Sprinkle chopped/cubed sirloin with salt, pepper and garlic powder to taste. Heat a large skillet over medium heat, spray with cooking spray and brown the seasoned beef. Transfer to a slow cooker.
Mix bouillon cube with hot water until dissolved, then mix in cornstarch until dissolved. Pour into the slow cooker with meat. Stir in sliced onion and peppers, stewed tomatoes, soy sauce, sugar, and salt.
Cover, and cook on High for 3 to 4 hours, or on Low for 6 to 8 hours. Serve over warm cooked rice. Enjoy!!