In a small food processor or blender combine chopped onion, lime juice, cumin, garlic, ground chipotle pepper, and salt. Cover and process or blend to form a thick paste.
Spread paste over both sides of steak. Using your fingers, rub paste into meat. Place steak in a resealable plastic bag set in a shallow dish; seal bag. Chill in the refrigerator for 2 to 8 hours.
For a charcoal or gas grill, grill steak on the rack of a covered grill directly over medium-hot heat about 4 minutes or until steak is lightly charred, turning once halfway through grilling time. Transfer steak to a cutting board; cover with foil. Let stand for 5 minutes. Coarsely chop steak.
Divide chopped steak among warm tortillas. Top with cabbage, tomatoes, red onion, and cilantro. Serve immediately.