Dice enough potatoes to cover bottom of skillet. Dice onion and brown together until tender. Chop about 1 or 2 cups leftover roast and place in skillet. Simmer until warm; add mixed peas and carrots. Heat thoroughly. If you have leftover brown gravy, pour this over vegetables. If not, use brown gravy mix plus 1 cup water. Heat to bubbling. Place canned biscuits on top. Put in preheated 425 degree F oven and bake until biscuits are golden.