Marinate: Whisk together lime juice, oil, garlic, cumin, brown sugar (or honey), black pepper, and salt. Coat the flank steak, cover, and refrigerate at least 30 minutes (up to 4 hours).
Preheat: Set the air fryer to 400°F (200°C) and preheat 3–5 minutes.
Cook the steak: Air fry at 400°F for 10–12 minutes, flipping halfway, until it reaches your desired doneness (130°F medium-rare, 140°F medium).
Rest: Transfer to a cutting board, tent with foil, and rest 5 minutes.
Cook the veggies: Toss peppers and onions with oil, salt, and pepper. Air fry at 380°F for 8–10 minutes, shaking halfway, until softened and charred.
Serve: Slice the steak against the grain. Serve with warm tortillas, pico de gallo, cheddar, guacamole, sour cream, and cilantro.