You Will Need
-20 white button or cremini mushrooms (10 ounces)
-1 tablespoon olive oil
-1 tablespoon butter
-1/2 cup chopped onion, half a medium onion
-2 tablespoons minced garlic, 4 to 5 cloves
-2 teaspoons chopped fresh rosemary or thyme
-2 ounces (56g) shredded cheese like sharp cheddar, mozzarella, Swiss, or a combination
-3 tablespoons breadcrumbs, try panko or homemade
-Salt and fresh ground black pepper
-1 tablespoon chopped fresh parsley or chives for serving, optional
Directions
1 Preheat the oven to 375°F (190°C).
2 Using a damp paper towel, brush all dirt from the mushrooms. Remove the mushroom stems. Cut away and discard the harder bits of the stems, then finely mince them.
3 Add the mushroom caps to a rimmed baking sheet or sizeable oven-safe skillet that fits the mushrooms in one layer. Toss them with a tablespoon of olive oil and lightly season with fresh ground black pepper. Arrange them cavity-side-up so that they are ready to be stuffed.
4 Melt the butter in a skillet over medium heat. Add the onions, chopped mushroom stems, and a pinch of salt. Cook, stirring occasionally, until the onions smell sweet and are soft, for about 5 minutes.
5 Add the garlic and herbs, then cook for another minute or until the garlic is fragrant. Before the garlic begins to brown, scrape the mixture into a bowl and set it aside to cool.
6 Add the breadcrumbs to the same skillet. Place it over medium-low heat, then cook, stirring, until lightly toasted, about one minute.
7 Stir the cheese into the onions and garlic mixture, then divide the filling between the mushroom caps. Sprinkle the toasted breadcrumbs on top.
8 Bake the mushrooms until the cheese has melted and the mushrooms have softened, 15 to 20 minutes.
9 Serve with chopped fresh herbs and freshly ground black pepper on top.