To cook a roast in the oven is pretty easy. You want to use 1 cup onions, 2 cups of cut potatoes, and 2 cups of cut carrots. Use 1 pack of onion soup mix. Place the roast in the roasting pan, then add all veggies and soup mix with about 3 cups of water. Put in pre-heated oven at 350 degrees for 1 hour.