Trim any fat from the steak, then slice into thin strips. Add to slow cooker. Then add garlic, salt, pepper, soy sauce, hoisin sauce and sugar to the slow cooker.
Cook on low for 4 hours, then switch to high, adding the diced tomatoes and sliced peppers.
In a small bowl, combine cornstarch and water and stir until smooth, then add to the slow cooker. Continue cooking covered in high for 15-20 minutes, until the sauce is thickened.
Add bean sprouts and onion, then serve immediately.