Season browned roast with salt and pepper; place in slow cooker. Top with onion, peppers, mushrooms and tomatoes.
Mix gravy powder with 1 tomato can of water. Pour over roast and veggies.
Cook 6 to 8 hours on high.
Mix 2 to 3 tablespoons cornstarch with 1/4 cup water. Stir in to thicken sauce.
Serve with mashed potatoes or over cooked noodles.