French Onion–Stuffed Mushroom Bites

Preheat oven to 325°F. Remove and discard stems from 24 mushrooms; use a spoon to scrape and discard gills. Toss the mushroom caps with 1 tablespoon oil and ¼ teaspoon salt on a large rimmed baking sheet. Arrange cap-side up and bake until tender, 10 to 15 minutes.

Meanwhile, heat 1 tablespoon butter and the remaining 1 tablespoon oil in a large skillet over medium heat. Add sliced onions and the remaining ½ teaspoon salt; cook, stirring often, until deeply golden brown, about 40 minutes. Stir in 1 tablespoon vinegar, scraping up any browned bits; cook until evaporated, about 1 minute. Add ½ cup broth and cook, stirring often, until the onions are glazed and most of the liquid has evaporated, about 3 minutes.

Fill each mushroom cap with about 2 teaspoons caramelized onions. Top the mushrooms with 1 cup Gruyère and sprinkle with 2 tablespoons breadcrumbs. Bake until the cheese is melted, about 10 minutes. Increase oven temperature to broil; broil until the cheese is bubbly and the breadcrumbs are golden, 3 to 5 minutes. Garnish with thyme, if desired.