Sprinkle the beef sirloin strips with garlic powder. Heat the vegetable oil in a large skillet over medium heat. Sear the beef strips for about 5 minutes on each side until browned.
In a separate container, dissolve the beef bouillon cube in hot water. Mix in the cornstarch until smooth to create a thickening agent for the sauce.
Transfer the seared beef strips to the slow cooker. Pour the bouillon and cornstarch mixture over the beef.
Add the chopped onion, green bell peppers, stewed tomatoes with their liquid, soy sauce, sugar, and salt. Stir to combine all the ingredients evenly.
Cover the crock pot and cook on High for 3 to 4 hours or on Low for 6 to 8 hours, depending on your schedule.
Once cooked, serve warm over rice, noodles, or mashed potatoes, enjoying the delightful flavors of the dish.