Combine all the steak marinade ingredients in a medium mixing bowl. Marinate the steak for 30 minutes.
Place the peppers, onions, garlic, cilantro, and marinated steak (with the marinade) in the slow cooker and cook on low for 8 hours or high for 4 hours.
Transfer the steak from the slow cooker to a sheet pan and broil for 4-5 minutes, until it begins to char. Slice thinly and return to the slow cooker to toss in the juices.
Serve the fajitas over warm tortillas or over rice.