In a bowl, combine avocado oil, lime juice, garlic, chipotle powder, coriander, cumin, and oregano. Stir to incorporate and set aside.
Using a fork, poke holes in the flank steak to let the marinade penetrate more quickly. Liberally season both sides with salt and pepper. Add the flank steak to a zip-top bag, pour in the marinade and massage to coat. Seal the bag, squeezing out as much air as possible, and refrigerate for 1 to 4 hours.
45 to 60 minutes before cooking, remove the steak from the fridge and let it come to room temperature. Preheat 2 tablespoons of avocado oil in a cast-iron skillet over medium-high heat. Add the steak and cook, flipping every 60 to 90 seconds, until the thickest part of the meat reads 140F to 145F on an instant read thermometer, about 8 to 9 minutes total, depending on the thickness of the steak.
Transfer steak to a board and let it rest at least 6 minutes before slicing against the grain.
Wipe the skillet clean and add 2 tablespoons avocado oil. Add the onion and red and green peppers and season with a pinch of salt. Cook, stirring often, until the vegetables have softened and slightly caramelized. Taste for seasoning and adjust salt as required.
Add romaine lettuce to individual serving bowls. Top with sliced steak and sautéed vegetables, and drizzle with compliant salsa or dressing of choice. Enjoy!