In a blender combine sauce ingredients (lime juice, orange juice, cilantro, chipotle pepper, garlic clove, salt, pepper and cumin). Pulse these items until pulverized.
Put flank steak in slow cooker, and pour sauce over the steak.
Slice bell peppers and onions in a thick slice, so they hold their shape after cooking all day.
Put bell peppers and onions on top of steak and sauce.
Set slow cooker to low, and cook for 8 hours.
Serve steak, bell pepper and onions on flour or corn tortillas, top with guacamole, cheese and sour cream to taste.
Notes
Nutritional values do not include tortillas and toppings