2(12-oz) bags frozen green beansdefrosted, rinsed and drained/patted dry
Cook mushrooms: Melt butter in deep skillet over medium-high heat. Add mushrooms and cook until softened, about 5 minutes. Stir in garlic and cook for one more minute.
Make sauce: Reduce heat to medium-low. Sprinkle flour evenly over mushroom mixture, stirring well for 1-2 minutes. Pour in milk and broth, stirring constantly to keep any lumps from forming. Stir in Worcestershire sauce, pepper and nutmeg. Season with salt to taste, then simmer until thickened, about 5 minutes, stirring often.
Prep: As sauce is simmering, place a rack in the lower-middle part of the oven. Heat oven to 350°F.
Finish assembly: Once thickened, remove gravy from heat. Stir in green beans and ½ - ¾ of ONE can of fried onions. Spread in shallow, greased 9x13-inch baking dish and top with fried onions to taste.
Bake: Bake casserole until bubbly, 15-25 minutes (depending on size of dish; frozen beans must be fully heated through before serving!). Allow to sit on counter for 5-10 minutes, then serve immediately.
Green beans: I use frozen, defrosted, drained green beans. If you’re using fresh, make sure to cook them first.
Mushrooms: White cremini mushrooms are what I prefer here, but brown button are fine, too.
Make sure you lower the heat before sprinkling the flour over the mushrooms, you do not want to burn it or your sauce will be ruined beyond repair.
Likewise, make sure your pan is not too hot when you pour in the liquids, else the flour will seize up too quickly and yield a lumpy sauce.
Blanching frozen green beans first to reduce the risk of any food-borne illness if making the casserole ahead. Then fully assemble, but do NOT add fried onion topping. Tightly wrap in plastic wrap and refrigerate for up to 3 days.Remove casserole from fridge, unwrap and let sit on counter as oven heats up. Add the fried onion topping and bake as directed, adding 5-10 extra minutes if needed. Again, it’s important for frozen vegetables to be fully cooked before consuming.Leftovers: Allow casserole to cool to room temperature, then wrap well and store in refrigerator for up to 4 days. Thoroughly reheat before serving.