Brown meat in skillet with small amount do oil, until browned on all sides. Remove to preheated crockpot.
Add about 1/4 cup water to hot skillet and stir, loosening bits of browned meat and juices. Pour over the meat and add onions, pepper, garlic, chillies, and drained tomatoes. Sprinkle with salt, pepper, chili powder, and cumin to taste.
Cook overnight until meat shreds easily with a fork.
Just before serving add cilantro to taste.
Serve with warm taco shells, shredded lettuce, sour cream, shredded cheese and salsa.