Prep Mushrooms: Preheat oven to 400°F (204°C). Wipe the dirt off of each mushroom. Pluck out the stems and chop them up finely. Place the mushroom caps on a parchment-lined baking sheet.
Flavor Base: Heat oil in a large saute pan over medium heat, then add onion, garlic, and thyme. Cook until onion is soft, about 3 minutes.
Add Stems: Add chopped mushroom stems to the pan of onions and continue cooking until they release their moisture, about 5 to 10 minutes.
Filling: Remove from heat and stir in all remaining ingredients. Carefully spoon a bit of filling into each hollowed out mushroom.
Bake: Bake for 15 minutes, or until golden brown.
Notes
Store leftovers in an airtight container in the fridge for up to 5 days. Reheat in the microwave, oven, or air fryer.