Preheat oven to 375°F. Separate the stems from the mushroom caps. Dice the stems.
24 button mushrooms
Heat the butter in a small frying pan over medium-low heat. Add the onions to the pan and fry until soft and golden brown (about 15 minutes). Add the diced mushroom stems and cook for 4 minutes, stirring often.
2 tablespoons butter, 1 yellow onion
Stir in the garlic and herbs. Cook for 1 minute. Remove from heat and season with salt and pepper, to taste.
1 garlic clove, 1 teaspoon dried herbs
Fill each mushroom with the stuffing and place them on a baking sheet.
Sprinkle grated Parmesan cheese over each mushroom.
¼ cup Parmesan cheese
Bake for 20-25 minutes or until the cheese browns a little. Allow to cool for 5 minutes or so before serving. Garnish with fresh thyme.
Video
Notes
Store leftover cooked mushrooms in an airtight container in the fridge for up to 4 days.
Reheat in the oven to avoid sogginess.
Make ahead instructions:
You can stuff the mushrooms and store them in the fridge (before baking) 1 day in advance. Make sure they are covered well. When it's time to serve, you can take them out of the fridge, pop them onto a baking sheet and into the oven.
You can freeze stuffed mushrooms before baking for up to 3 months. First, freeze them separately on a baking sheet and then transfer to an airtight container or freezer bag and back into the freezer. You can bake them from frozen, but keep in mind they will need a little more cooking time.