Slice beef across grain into long, thin strips (about 1/8-inch thick). In hotel pan, combine soy sauce, lime juice, chopped onions, garlic and pepper flakes; add meat strips. Cover and refrigerate at least 20 minutes or up to 24 hours. Drain and discard marinade.
For each serving, to order: Grill 6 ounces beef. Warm 3 tortillas. Serve beef on sizzle platter; accompany with tortillas and toppings.