French Onion Soup Stuffed Mushrooms

Preheat oven to 375°F (190°C). Lightly grease a baking sheet.

Remove the stems from the mushrooms. Chop the mushroom stems finely and set aside.

In a large skillet, melt butter and olive oil over medium heat. Add sliced onions and cook, stirring occasionally, until softened and deeply caramelized, about 20-25 minutes. Stir in the chopped mushroom stems during the last 10 minutes of caramelizing.

Add brown sugar (if using), thyme, salt, and pepper to the onions. Deglaze the pan with sherry or wine, scraping up any browned bits from the bottom. Cook until the liquid is mostly evaporated, about 2-3 minutes.

In a small bowl, combine the grated Gruyere cheese and breadcrumbs.

Stuff each mushroom cap generously with the caramelized onion mixture, then top with the Gruyere cheese and breadcrumb mixture.

Place the stuffed mushrooms on the prepared baking sheet and bake for 15-20 minutes, or until the mushrooms are tender and the cheese is melted and golden brown.

Garnish with fresh parsley and serve immediately.