Combine garlic powder and black pepper in a medium bowl. Cut beef across the grain into 1 1/2 inch long and 1 inch wide cubes. Mix with garlic powder/black pepper mixture and set aside. Slice bell peppers and onion into cubes. Dissolve bouillon in hot water and mix in cornstarch until dissolved.
Heat a wok or large skillet over medium heat. Heat the oil to hot and brown the seasoned beef cubes. Transfer stir fried meat to a crock pot. Give a quick stir to bouillon mixture and pour into the slow cooker.
Stir in onion, bell peppers, stewed tomatoes, soy sauce, sugar and salt. Cover and cook on High for 3 to 4 hours or cook on Low for 6 to 8 hours. Remove and serve crock pot pepper steak with rice or noodles.