Make the Fried Onions (Can be done up to 6 hours in advance)

• Heat oil to 350°F. Use a deep-fryer or sturdy skillet on your stove (only fill the skillet about halfway).
  Vegetable oil
• In a medium bowl, add the egg to the milk and gently mix.
  1½ cups milk, 1 large egg
• In a separate pan, mix ½ cup of the flour with ½ cup of the breadcrumbs and salt. (You'll use the rest of the flour and breadcrumbs as you continue to bread the onions).
  1 cup all-purpose flour, 1 cup bread crumbs, 1 teaspoon Kosher salt
• Working in batches, add the onions to the milk/egg mixture, then dredge in flour. Place on a large platter or plate. Repeat. You'll most likely need to add another ½ cup of flour and another ½ cup of breadcrumbs (with a pinch of salt) to finish up all the onions.
  2 medium onions
• Fry the onions until golden, about 4 minutes per batch. Drain on paper towels. Sprinkle with a little more salt.
• Prepare the Casserole
• Meanwhile, bring a pot of water to a boil.
• Add the beans, and cook for 8 minutes for al dente beans and 20 minutes for tender beans. Drain and set aside.
  1½ lb green beans
• Preheat oven to 400°F.
• Melt the butter in a large skillet over medium-high heat.
  4 tablespoon butter
• Add the mushrooms. Sauté them, stirring often, until mushrooms start to release their liquid, about 10 minutes. If, after 5 to 8 minutes, the mushrooms seem dry, add another tablespoon of butter. Cook until very soft. Sprinkle with a pinch of salt and black pepper.
  16 oz mushrooms
• Add the garlic to the mushrooms and cook for another 1 minute. Add the flour and stir to coat the mushrooms. Cook for 2 minutes, stirring occasionally.
  2 cloves garlic, ⅓ cup all-purpose flour
• Whisk in the stock and cream and continue whisking until most of the lumps are gone, about 2 minutes. Stir in the Worcestershire sauce, bring to a simmer, and cook, stirring often, until thickened, about 4 to 6 minutes. Stir in the salt, pepper, and nutmeg. Remove from the heat.
  2½ cups chicken stock, 1 cup heavy cream, 1 tablespoon Worcestershire sauce, 1½ teaspoon Kosher salt, ½ teaspoon black pepper, 1 teaspoon nutmeg
• Stir in about ⅓ of the fried onions and all of the green beans. Mix until well combined. If not using a cast iron skillet, transfer the mixture to a baking dish.
• Bake for 20 minutes, adding the remaining onions on top of the casserole in the last 5 minutes of baking (after 15 minutes). Remove and let rest for 5 to 10 minutes before serving. Or, cover, and serve within 30 minutes.