In a bowl, whisk together lime juice, vegetable oil, minced garlic, cumin, brown sugar (or honey), black pepper, and salt. Coat the flank steak in the marinade, cover, and refrigerate for at least 30 minutes (or up to 4 hours for best flavor).

Remove the steak from the marinade and place it in the air fryer basket. Air fry at 400°F for 10-12 minutes, flipping halfway through, or until the steak reaches your desired doneness (130°F for medium-rare, 140°F for medium).

Add the sliced bell peppers and onion to the air fryer basket. Drizzle with 2 tbsp vegetable oil, toss to coat, and air fry at 380°F for 8-10 minutes, shaking halfway through, until softened and slightly charred.

Slice the cooked steak against the grain. Serve with air-fried bell peppers and onions, and your choice of pico de gallo, shredded cheddar cheese, guacamole, sour cream, and fresh cilantro.