Heat oil in a large skillet over medium heat. Add steak strips, and quickly brown on both sides, adding garlic while the steak cooks. Transfer steak and its juices to a slow cooker. Add onion, soy sauce, salt, pepper and sugar. Cover, and cook on Low until the beef is fork tender, 6 to 8 hours.

One hour before the end of the cooking time, add green peppers. Stir cornstarch and cold water together in a small bowl; pour into the slow cooker during the last few minutes, and cook until the sauce has thickened.