Place frozen pierogies in a 2 qt. baking dish, set aside. In a large saucepan, cook onion in hot oil for 2-3 minutes or just until tender. Stir in sliced sausage, mushrooms, and garlic. Cook and stir 3 minutes more. Add spinach, cream cheese, broth, salt and pepper. Cook and stir over medium heat to melt cream cheese. Pour the sausage mixture over pierogies. Sprinkle with mozzarella cheese. Bake, uncovered in a 400 degree oven for about 30 minutes or until lightly browned and heated through.