Preheat your oven to 375°F and line a baking sheet with parchment paper for easy cleanup. Clean mushroom caps with a damp cloth and gently remove the dark gills using a small spoon, creating a small cavity for filling. Pat dry thoroughly—excess moisture causes steaming instead of roasting.

Heat butter in a large skillet over medium heat and add diced onion, cooking until translucent and fragrant, about 4 minutes. Add minced garlic and cook for exactly 1 minute more, stirring constantly to prevent burning. This blooming technique releases essential oils that build flavor depth throughout your creamy stuffed mushrooms filling.

Remove the pan from heat and let cool for 2 minutes, then fold in softened cream cheese, grated Parmesan, chopped parsley, thyme, salt, and pepper. Stir until completely combined with no visible cream cheese streaks remaining. Add chicken broth one tablespoon at a time until the mixture reaches a spreadable consistency.

Brush mushroom caps lightly with olive oil on both sides and arrange gill-side up on your prepared baking sheet. Spoon filling generously into each mushroom cavity, mounding slightly and filling to capacity. This ensures maximum creaminess in every bite of your stuffed baked mushroom creation.

Mix panko breadcrumbs with 1 tablespoon melted butter and shredded mozzarella, then sprinkle evenly over each filled mushroom. Bake for 18-20 minutes until caps are tender and the topping turns golden brown. Remove from oven and let rest for exactly 2 minutes before serving to allow the filling to set slightly without cooling completely.