Sprinkle garlic powder onto both sides of beef strips. Heat the vegetable oil in a large skillet over medium heat, add the beef and cook until 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, stir or whisk until the cornstarch is dissolved. Pour over the meat and vegetables in 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.