Preheat the Oven: Preheat your oven to 375°F (190°C).
Prepare the Mushrooms: Clean the mushrooms with a damp cloth, remove the stems, and set the caps aside. Chop the stems finely for the dip mixture.
Cook the Mushroom Stems: In a medium skillet, heat the butter and olive oil over medium heat. Add the chopped mushroom stems, onions, and garlic. Sauté for about 4-5 minutes until the mushrooms release their moisture and the onions become translucent.
Make the Dip: In a large bowl, combine the sautéed mushrooms and onions with the cream cheese, sour cream, Parmesan, mozzarella, breadcrumbs, salt, and pepper. Stir everything together until the mixture is smooth and creamy.
Stuff the Mushrooms: Spoon the dip mixture into each mushroom cap, filling them generously. If desired, top with a little extra mozzarella or Parmesan for a cheesy finish.
Bake: Place the stuffed mushrooms on a baking sheet lined with parchment paper. Bake for 15-20 minutes or until the mushrooms are tender and the tops are golden brown and bubbly.
Garnish and Serve: Once baked, remove from the oven and let cool for a few minutes. Garnish with fresh herbs like parsley or thyme. Serve warm with crackers, crusty bread, or fresh veggies for dipping.
