Prepare mushrooms by removing stems. Chop stems finely and set aside. Wipe mushroom caps clean with a damp paper towel.
In a large skillet, melt 4 tablespoons of the butter on medium heat. Add caps and toss gently to coat; sauté for about 3 minutes. Place on a lightly buttered baking dish.
Melt the remaining butter in the skillet on medium heat. Add the onions and the reserved mushroom stems; sauté for 2 minutes. Add bread crumbs, cheese, parsley, nuts, salt and pepper; stir lightly.
Spoon filling into mushroom caps. Bake in a preheated 350 degree oven until hot, about 20 minutes. Serve hot.
Serves 10 or more