Give earthy mushrooms the au gratin treatment with this rich side that's perfect for chillier nights.
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.
We used frozen pearl onions, but you could use fresh instead. To peel, place them in boiling water for one minute, and then let them cool slightly before slipping off their papery skins.