Salisbury Steak Casserole

Name: Salisbury Steak Casserole
Prep time: 5 min
Cook time: 20 min
Total time: 25 min
Cuisine: American
Ingredients: 1/2 cup butter (divided use); 1.5 pounds ground beef (lean 90/10 or better); 1 cup onions (chopped); 1 ounce onion soup mix; 1/2 cup water; 10.5 ounce cream of mushroom soup; 4 cups mashed potatoes (get at the deli to save time); black pepper to taste; 2 stalks green onions (garnish)