Open Faced Roast Beef

Ingredients

• ["2 lb Rare Deli Roast Beef (cheap is fine!)", "1 Onion, chopped", "1 packages Mushrooms (whatever's on sale but they should be sliced and this is not a required ingredient)", "4 envelope Low-sodium beef gravy mix", "1 cup Beef broth, divided", "1 tbsp Corn starch", "1 tsp Cumin", "1 tbsp Garlic", "1 as needed Potatoes (I make my own but instant is fine if you prefer)", "1 tsp Worchestershire sauce", "5 lb Potatoes", "1/2 cup Real butter (seriously, have you SEEN the ingredients in that supposed \"healthy\"" crap?)""