Green Bean Casserole Stuffed Mushrooms

Preheat oven broiler and move rack about 6 inches from heat. Wipe the mushrooms clean and pop out the stems, using a paring knife to remove any stem stuck in the cap. Chop stems small and set aside. Place caps dome up on a nonstick (or lightly oiled) rimmed baking sheet. Spray or brush tops with oil and sprinkle with pepper. Broil about 5 minutes, until caps just begin to soften and release a little liquid. Set oven to 375ºF and set mushrooms aside.

Heat 1 tablespoon butter or vegetable oil in a 10-inch skillet over medium heat. Add mushroom stems and sauté, stirring infrequently, until browned and tender, 7–10 minutes. Scoop out of the skillet and into a mixing bowl, leaving extra oil in the skillet. If no oil remains, add ½ tablespoon. Raise heat to medium high and add frozen beans. Cook until tender and any water has evaporated, about 5–10 minutes. Add to bowl with the mushroom stems. Carefully wipe out skillet with a damp cloth.

Heat milk until warm, about 2 minutes in the microwave at 50% power. Melt butter in the skillet over medium low, then add shallots. Cook a few minutes, until tender but not browned, then sprinkle flour and mustard powder into the pan. Stir constantly for 2–3 minutes, until flour is no longer grainy and has darkened slightly. Reduce heat to low, then slowly whisk in milk. Raise heat gradually to bring to a simmer while whisking. Adjust heat to maintain a gentle simmer, stirring frequently, until thickened, about 5 minutes. Off heat, stir in ⅓ cup of the Parmesan, ¾ teaspoon salt, white pepper (a little more if using black), paprika, and parsley. Adjust salt and pepper to taste.

Turn the mushroom caps face up. Mix half the sauce with the bean mixture. Pile into the mushroom caps, packing it into a mound. Bake for 15 minutes, until mushrooms are juicy and tender. Mix remaining Parmesan with the fried onions. Remove tray from oven, sprinkle mushrooms with fried onion mixture, and bake or broil another few minutes, until browned. Cool slightly before sprinkling with more parsley and serving warm.