Step 1: Preheat your oven to 375°F (190°C) to ensure even cooking.
Step 2: Gently clean the button mushrooms with a damp cloth and remove the stems.
Step 3: Finely chop the mushroom stems, onion, and garlic.
Step 4: In a skillet over medium heat, melt the butter, add onion and garlic, and sauté until soft. Stir in chopped mushroom stems and thyme, cooking for an additional 3-4 minutes.
Step 5: In a mixing bowl, combine the sautéed mixture with bread crumbs and Parmesan cheese, seasoning with salt and pepper.
Step 6: Stuff each mushroom cap with the prepared filling, pressing down gently.
Step 7: Place the stuffed mushrooms in a baking dish and drizzle with olive oil.
Step 8: Bake for 20-25 minutes until golden brown and tender.
Step 9: Allow to cool slightly before serving warm.