Add oil to skillet and brown beef, sprinkle with garlic powder, then quickly sauté vegetables.
Mix bouillon with hot water, then add 1 tablespoon of cornstarch until dissolved. Pour into slow cooker with meat. Stir in stewed tomatoes, soy sauce, and sugar. Set veggies aside in refrigerator for a few hours.
Cover and cook on low 5 hours, add vegetables at hour 3 and take a few tablespoons of sauce out of crockpot, put into a bowl and mix with remaining 1 tbs. of cornstarch. Add back to crockpot and stir. This will thicken the dish up. Serve over rice.