Preheat the grill to high heat (about 450°F). Place a metal grill basket on the grill to preheat.
Toss the sliced onion and bell peppers with the avocado oil until evenly coated. Season with 1 tablespoon of the Fiesta Rub.
Season both sides of the flap steak with the remaining 2 tablespoons of Fiesta Rub.
Prepare your tortillas and toppings so everything is ready to serve when the steak comes off the grill.
Place the vegetables in the preheated grill basket and the steak directly on the grill grates. Grill with the lid open, stirring the vegetables and flipping the steak every 3-5 minutes. Cook until the vegetables are lightly charred and tender and the steak reaches your preferred internal temperature: Rare 125°F; Medium-rare 135°F; Medium 145°F.
Remove the steak and vegetables from the grill. Let the steak rest for 5-10 minutes. Meanwhile, warm the tortillas on the grill or in a cast iron skillet. Wrap them in foil to keep warm.
Slice the steak thinly against the grain. Arrange the steak and vegetables in the hot cast iron skillet and serve immediately with the warm tortillas and your favorite toppings.
Recipe Tips
Steak temperatures: Rare 125°F; Medium-rare 135°F; Medium 145°F.
