Preheat your oven to 400°F (200°C). Line a large rimmed baking sheet with parchment paper or brush a cast iron skillet lightly with olive oil.

Gently wipe the mushroom caps with a damp paper towel to remove any peat (never submerge in water). Pop the stems out of the mushroom caps. Finely mince half of the stems for the filling (discard or compost remaining stems).

In a small skillet, melt 1 tablespoon butter over medium heat. Sauté the minced mushroom stems and minced garlic for 2 to 3 minutes until softened and fragrant. Remove from heat and allow to cool slightly.

In a medium bowl, combine softened cream cheese, sautéed garlic-stem mixture, 1/4 cup grated Parmesan cheese, chopped parsley, thyme, onion powder, salt, and black pepper. Stir until smooth and creamy.

Toss the hollow mushroom caps lightly with olive oil and arrange cavity-side up on the prepared baking sheet. Using a small spoon, mound the cream cheese filling generously into each mushroom cap.

In a small bowl, toss panko breadcrumbs, melted butter, and remaining Parmesan cheese together. Sprinkle a generous pinch over each stuffed mushroom.

Bake for 18 to 20 minutes until the mushrooms are tender, the filling is bubbling hot, and the breadcrumb topping is beautifully golden brown and crispy. Let cool for 5 minutes before serving on a platter.