Start by preparing the marinade. Combine vegetable oil, lime zest, lime juice, smashed garlic, quartered onion, chipotles in adobo, cumin, chili powder, oregano, salt, and black pepper in a blender. Blend until smooth. Place your flank steak in a large bowl or plastic bag and pour the marinade over it. Make sure the steak is well-coated. Cover and refrigerate for at least 1 hour or up to overnight for maximum flavor.

While the steak marinates, slice your onions and bell peppers. Heat a cast iron skillet over medium-high and drizzle with olive oil. Once the oil shimmers, add the veggies in a single layer. Allow them to char slightly for about 2-3 minutes before tossing. Cook until they are tender-crisp yet vibrant in color, then set aside.

Remove the steak from the marinade and pat it dry with paper towels. Heat the skillet over medium-high heat. Add the steak and sear for about 3-4 minutes on each side for medium-rare. Use a meat thermometer to ensure it reaches 130°F for the perfect texture. Once cooked, transfer the steak to a cutting board and let it rest for 10 minutes.

Once rested, slice the steak against the grain into thin strips. Warm the tortillas on a skillet for 20-30 seconds on each side. Fill each tortilla with sliced steak, fajita veggies, and your choice of toppings.