Place oil in a skillet and bring to a medium high heat.
Place beef strips in the saucepan and brown just long enough to get the outside crusted and caramelized but do not cook all the way through.
Do not overload load the pan down or your meat will steam not brown. If needed cook in batches.
Transfer to slow cooker.
Mix together soy sauce, garlic, onions, sugar, salt, pepper, and ginger.
Pour over meat.
Cover and cook on Low 5-6 hours.
Add green peppers and tomatoes.
Cook for 1 additional hour.
Combine water and corn starch to make a paste.
Stir into slow cooker.
Cook on High until thickened, about 10 mins.
Serve over rice or noodles