Sprinkle garlic powder onto both sides of the beef strips. Heat the vegetable oil in a large skillet over medium heat, add the beef, and cook until the outside is browned, beef does not need to be fully cooked.
Transfer to a slow cooker. Stir in onion, bell peppers, and fire-roasted tomatoes.
Mix bouillon powder/cube with hot water until dissolved. Add the soy sauce, sugar, and cornstarch, and stir or whisk until the cornstarch is dissolved. Pour over the meat and vegetables into the slow cooker.
Cover, and cook on High for 3 to 4 hours, or on Low for 5 to 7 hours.
Serve over rice, mashed potatoes, or egg noodles if desired.