Preheat the oven to 375?F (190?C).

Using a damp paper towel, brush all dirt from the mushrooms. Remove the mushroom stems, cut away and discard the harder bits, then finely mince them.

Add the mushroom caps to a rimmed baking sheet or oven-safe skillet, toss them with olive oil, and lightly season with black pepper. Arrange them cavity-side-up.

Melt the butter in a skillet over medium heat. Add the onions, chopped mushroom stems, and a pinch of salt. Cook until the onions are soft, about 5 minutes.

Add the garlic and herbs, then cook for another minute or until fragrant. Transfer the mixture to a bowl and set aside to cool.

In the same skillet, toast the breadcrumbs over medium-low heat, stirring until lightly toasted, about one minute.

Stir the cheese into the onions and garlic mixture, then divide the filling between the mushroom caps. Sprinkle toasted breadcrumbs on top.

Bake the mushrooms until the cheese has melted and the mushrooms have softened, 15 to 20 minutes.

Serve with chopped fresh herbs and freshly ground black pepper on top.