Slice the onion and bell peppers and toss with 1 tablespoon of the marinade.
Slice the beef into ½-inch strips. Toss with the remaining marinade and refrigerate for at least 15 minutes.
Preheat the air fryer to 400°F.
Place the vegetables in the air fryer basket in a single layer and cook 6-7 minutes or just until tender crisp. Remove from the air fryer and set aside.
Place the beef in the air fryer basket in a single layer and cook 4-6 minutes or just until cooked. Add peppers back to the air fryer and cook 1 minute more.
Serve in warmed tortillas with toppings.
Notes
Steak does not need to be cooked to well done, it's ok if it's a little bit pink inside. Check the steak early to ensure it doesn't overcook.