Season the halibut fillets with salt and pepper.
4 6 oz halibut fillets, Salt and freshly ground black pepper
In a large skillet with a lid (non-stick is preferable), heat 2 tablespoons of the oil over medium-high heat. Once shimmering, add the butter and swirl until melted.
2 tbsp olive oil, unsalted butter
Add the halibut and sear until golden brown on one side, about 3 minutes (you may need to do this in batches). Transfer to a plate.
Add the onion and cook until soft, about 4 minutes. Add the garlic and cook, stirring often, for another 30 seconds.
½ cup red onion, 4 cloves garlic
Stir in the rosemary, thyme, and tomatoes and cook for another 1 to 2 minutes, stirring occasionally.
1 tsp rosemary, 1 tsp thyme, ¾ cup cherry tomatoes
Add the broth and beans and bring to a simmer.
⅔ cup chicken broth, ½ cup cannellini beans
Working in batches, add the spinach to the skillet and cook until completely wited down. Repeat with remaining spinach.
5 oz spinach
Stir in the lemon juice, olives, red pepper flakes, ½ tsp salt, and ¼ tsp pepper. Stir to combine.
2 tbsp lemon juice, ½ cup green olives, Pinch red pepper flakes
Turn down the heat to medium-low and nestle the fillets into the vegetables and sauce. Cover and cook for about 3 minutes, or until fillets are completely cooked through (internal temperature of 130 to 135°F).
Ladle the vegetable and bean sauce into shallow bowls and then nestle a fillet in each. Drizzle more sauce over the top. Serve at once.