Slice the flank steak against the grain into strips about 1/4 inch thick. This is the single most important step for tender fajita meat. Cutting with the grain makes the steak chewy. Toss the steak strips with the fajita seasoning and 1 tablespoon of the olive oil in a bowl and set aside while you prep the vegetables.

Slice the bell peppers and onion into strips of similar thickness so they cook evenly. Mince the garlic and set everything near the stove because this recipe moves fast once the heat is on.

Set your cast iron skillet over high heat and let it get genuinely hot, about 2 minutes. You want it hot enough that a drop of water evaporates immediately on contact. Add the remaining tablespoon of oil and swirl to coat.

Add the peppers and onion to the skillet. Season with the salt and cook over high heat for 4 to 5 minutes, stirring every minute or so. You want them slightly charred at the edges and tender but still with a little bite. Add the garlic in the last minute and stir to keep it from burning. Transfer the vegetables to a plate.

With the skillet still on high heat, add the seasoned steak in a single layer. If your skillet cannot hold all the steak without crowding, cook it in two batches. Crowding drops the pan temperature and you will end up steaming the meat instead of searing it. Cook for 2 to 3 minutes per side without moving it much, letting a brown crust form.

Return the peppers and onion to the skillet with the steak. Squeeze the lime juice over everything and give it a quick toss to combine. Taste and add a pinch more salt if needed.

Serve immediately in warm tortillas topped with sour cream, salsa, and sliced avocado. Set lime wedges on the side for squeezing.