Scalloped Mushrooms

Preheat oven to 350 degrees. Heat 1 tablespoon butter in a large cast-iron or ovenproof skillet over medium-high heat. Add onions, and cook until soft and just starting to brown, about 5 minutes. Transfer to a large mixing bowl.

Working in four batches, melt 2 tablespoons butter in same skillet; add one-quarter of the mushrooms, tossing to coat evenly with butter. Cook until mushrooms have released their juices and most of the juices have evaporated, about 5 minutes. Transfer to bowl with onions. Repeat process with remaining butter and mushrooms.

Add heavy cream, 1/2 cup Parmesan, salt, and pepper to bowl; stir until combined. Return the mixture to skillet or transfer to a large baking dish. Sprinkle breadcrumbs and remaining 1/2 cup Parmesan over the top.

Bake until bubbling and nicely golden on top, about 25 minutes. Remove from oven. Serve hot.