Melt butter in a large skillet over low heat; brush mushroom caps with melted butter and place onto a baking sheet. Raise heat to medium; cook and stir chopped mushroom stems, green onion, and garlic in the butter remaining in the skillet for 3 minutes. Lightly stir bread crumbs, Cheddar cheese, pecans, salt, and black pepper into the mixture. Generously fill the mushroom caps with stuffing, piling it high.
Bake in the preheated oven until stuffing is hot, the cheese has melted, and mushrooms are cooked through, 20 to 30 minutes.