Seared Haddock with Mushroom Agrodolce

This is one seafood dish that we think is nothing short of perfection.

Agrodolce is an Italian sauce which means sweet and sour combined.  The whole onions and mushrooms are cooked down in an amazing sauce of balsamic vinegar and sugar. Sear up a beautiful white fish, like haddock, and you’ve got a dish incredibly flavorful and on the table in about 30 minutes!

A white plate filled with seared haddock topped with mushroom agrodolce sauce next to a glass of white wine.

How to Make Seared Haddock with Mushroom Agrodolce

This is one of those dishes that is so deep in flavor, it feels as though it must have taken hours to achieve such a robust sauce.

But in reality, it only takes about 30 minutes from start to finish.

 

Choosing the Right Type of Onion

Cipollini onions are often cooked in an agrodolce sauce in Italy.

These types of onions are perfect for this dish and are often found in the produce section of many well-stocked markets. Other types of onions that would work are boiling onions or pearl onions.

EXPERT TIP: To peel the skin from the onions, simply drop them into a small pot of boiling water for 1 minute. Drain them and then snip the ends off on both sides, then simply use your fingers to squeeze the onion out.  Watch out, though, they’ll squirt across the kitchen, if you’re not careful!

Cipolinni onions in boiling water and then a hand removing the skin from the boiled onions.

Mushrooms pair wonderfully against the onions.

Just like the onions, we love to leave them whole.

Sauté them in a large skillet with 3 tablespoons of olive oil and a little salt and pepper until lightly browned and soft, about 12 minutes.

EXPERT TIP: After you stir in the balsamic vinegar and the sugar, you’ll want to stir consistently until the sauce thickens somewhat and becomes almost like syrup. Don’t overcook it, though. As soon as it starts to thicken, remove from heat. This process usually only takes a couple of minutes.

Whole mushrooms being sautéed in a saucepan with balsamic vinegar.

How To Cook Haddock

After you’ve made the mushroom agrodolce, transfer it to a bowl.

Add a little more oil to the skillet if it’s a bit dry and then heat to medium-high heat.

Sear the haddock, skin side up, for 4 minutes. No need to flip. You’ll then transfer the partially seared fish to a baking dish and then pour the mushroom agrodolce over it..

Cooked mushrooms in an agrodolce sauce being poured over seared haddock in a baking dish.

Besides being incredibly flavorful and satisfying on so many levels, the dish is beautiful in presentation, too.

Agrodolce is great with all kinds of proteins, and over pasta, but we think it pairs beautifully with a flaky white fish, such as haddock.

EXPERT TIP: In North America, haddock can be found in the seafood section of many well-stocked markets. We get our frozen at Whole Foods. Although fresh is almost always best, we still get amazing results with frozen. Other perfectly suitable substitutes for haddock are cod, flounder, sole, or halibut.

A baking dish filled with seared haddock with mushroom agrodolce sauce and sprigs of thyme on top.

When to Serve Seared Haddock with Mushroom Agrodolce

This dish is so wonderful, we serve it year-round, but there is definitely an autumnal feel to it, so it’s especially wonderful in the fall.

The mushroom agrodolce serves as a wonderful side to the fish. We recommend serving it with Maple-Glazed Brussels Sprouts or Best Mashed Potatoes.

This recipe is perfect for serving on a weeknight, but also elegant enough, that it’s spectacular when entertaining.

A spoon pouring agrodolce sauce onto a plate of seared haddock.

And did we mention how quickly this restaurant-quality dish comes together?

In about 30 minutes it’s on the table!

EXPERT TIP: The mushroom agrodolce sauce can be made up to 24 hours in advance. Simply store it in an air-tight container in the refrigerator. Gently reheat in a skillet and then transfer to a bowl and then sear the haddock in the same skillet.

This is a true show-stopper of a dish. You won’t believe something this delicious can be so simple to prepare!

A white plate filled with filet of seared haddock topped with mushroom agrodolce sauce.

Ready to make the best seafood dish this side of Rome? Go for it!

And when you do, be sure to take a photo of it, post it on Instagram, and tag @HowToFeedaLoon and hashtag #HowToFeedaLoon!

A white plate filled with filet of seared haddock topped with mushroom agrodolce sauce.

Seared Haddock with Mushroom Agrodolce

This Seared Haddock with Mushroom Agrodolce is divine. The slightly sweet, slightly tart sauce compliments the creaminess of the haddock perfectly. Don't worry, this isn't a 'fishy' tasting dish at all. Simply amazing, and comes together in about 30 minutes. Perfection!
5 from 1 vote
Print Pin Rate
Course: Seafood
Cuisine: Italian
Keyword: agrodolce sauce recipe, easy seafood recipe, How to cook haddock
Prep Time: 10 minutes
Cook Time: 20 minutes
Total Time: 30 minutes
Servings: 4
Calories: 315kcal

Equipment

  • Skillet and baking dish (or cast-iron skillet)

Ingredients

  • 5 tbsp extra-virgin olive oil divided
  • 6 cipollini onions about 1 cup, or boiling or pearl onions
  • 8 oz mushrooms white button, stems trimmed
  • Kosher salt
  • black pepper freshly ground
  • ½ cup balsamic vinegar
  • tbsp sugar
  • 1 lb haddock cut into 4 individual fillets
  • 6 sprigs thyme fresh, plus more for garnish, chopped

Instructions

  • Heat oven to 400°F.
  • Bring 2 cups of water in a medium pan to a boil over high heat. Add the onions and cook for 1 minute. Drain. When cool enough to handle, use a knife to snip off the ends of the onions. Use your fingers to gently squeeze the onion out of the skin. Set aside.
  • Heat 3 tablespoons of oil in a large saucepan/skillet over medium-high heat. Add onions and mushrooms and a good pinch of salt and pepper and sauté, stirring occasionally, until nicely browned and soft, about 10 to 12 minutes.
  • Add vinegar and sugar to the skillet and simmer. Stir frequently with a wooden spoon to scrape any browned bits stuck to bottom of the skillet. Cook until sauce thickens slightly and comes almost like syrup, just a few minutes. Transfer onions, mushrooms, and sauce to a bowl, and set aside.
  • Rub haddock fillets with remaining 2 tablespoons oil, then with salt and pepper. If the skillet is dry, add another tablespoon of olive oil and heat over medium-high heat.
  • When the skillet is hot, add fillets, skin side up, and sear until lightly browned on the bottom, about 4 minutes. Remove from heat and place the fish, seared side up, in a baking dish. Pour the mushroom agrodolce sauce all over the fish and top with the sprigs of thyme.
  • Place in oven and roast until fish is cooked through, about 10 minutes. Remove from oven and discard the thyme sprigs on top. Plate and garnish with a pinch of chopped fresh thyme. Serve at once.

Video

Notes

Pearl onions or boiler onions are perfectly fine substitutions for cippolini.  Frozen pearl onions can be used in a pinch.  Let thaw before adding them to the skillet.
We love leaving the mushrooms whole, but they can be quartered or sliced, if desired.  Any variety of mushroom will work for this dish but we think good ole white button work wonderfully.
Substitutions for haddock are cod, grouper, halibut, or sole.  Any kind of white, flakey fish will do great.  We find haddock in the frozen seafood section at Whole Foods.   Fresh is always best, but the frozen filets from Whole Foods are still delicious. 
The sauce can be made up to 24 hours in advance.  The sauce can also be frozen for up to 1 month.  The finished dish will keep in the refrigerator for a couple of days, although, fresh will always be best.  
This is great served with braised Brussel sprouts or mashed potatoes.  It's also excellent over risotto. 

Nutrition

Calories: 315kcal | Carbohydrates: 16g | Protein: 21g | Fat: 12g | Saturated Fat: 3g | Cholesterol: 61mg | Sodium: 254mg | Potassium: 606mg | Fiber: 1g | Sugar: 12g | Vitamin A: 65IU | Vitamin C: 5mg | Calcium: 31mg | Iron: 1mg
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POST UPDATE: This recipe was originally published June 2016, but was updated with improved tweaks to the recipe and new tips and photography and a fabulous new video in October 2020!