Thinly slice the flank steak and place it along with the sliced onion in the base of a slow cooker.
Mix the salsa, cilantro, lime juice, garlic, chili powder, cumin, and salt in a small bowl and pour over to evenly cover. Cook on the lowest setting for 10 hours.
One hour before the beef is done cooking, add the sliced bell peppers to the slow cooker so that they may soften slightly.
Serve the beef, onion, and peppers in warmed flour tortillas with any toppings you like.