Preheat oven to 375°.

In a 10-inch cast-iron skillet, place bread crumbs. Cook over medium-high heat until lightly browned, approximately 3 minutes, stirring frequently. Remove from heat.

In a small bowl, combine bread crumbs, cheese, thyme, garlic, salt, and pepper. Add 2 tablespoons melted butter; toss well. Spoon about 1½ teaspoons bread crumb mixture into each mushroom. Place mushrooms in skillet. Drizzle with remaining 2 tablespoons butter.

Bake until mushrooms are tender but still hold their shape, approximately 15 minutes. Let cool for 10 minutes before serving.