Mix the marinade: Whisk the lime juice, olive oil, garlic, cumin, chili powder, 1 teaspoon of the salt, the black pepper, and the cayenne if using in a bowl. Pour it over the flank steak in a zip-top bag and press the air out.

Marinate 30 minutes to 4 hours: Refrigerate for at least 30 minutes and no more than 4 hours. Past that the lime juice starts breaking down the surface of the steak and the texture turns mealy.

Char the peppers and onions: Heat 1 tablespoon of the vegetable oil in a 12-inch cast iron skillet over medium-high heat until it shimmers. Add the onion and bell peppers and cook for 6-7 minutes, stirring only every minute or so, until they're crisp-tender with blackened edges. Season with the remaining 1/2 teaspoon salt and move them to a plate.

Sear the steak: Pull the steak from the marinade and pat both sides dry, then discard the marinade. Add the remaining 1 tablespoon oil to the same skillet over high heat and lay the steak in. Sear 4-5 minutes per side, until a thermometer in the thickest part reads 130-135°F for medium-rare. The USDA recommends cooking whole cuts of beef to 145°F followed by a 3-minute rest, which lands you at medium.

Rest the steak: Move the steak to a cutting board and let it rest 10 minutes, well past the 3-minute rest the USDA calls for. Cutting it early dumps the juice onto the board instead of leaving it in the meat.

Warm the tortillas: While the steak rests, warm the tortillas 20-30 seconds per side in the dry skillet, or stack them in foil and warm at 300°F for 10 minutes.

Slice against the grain: Look for the long fibers running the length of the steak and slice straight across them, about 1/4 inch thick, holding the knife at a slight angle. Every slice should be short across, not stringy.

Toss and serve: Return the sliced steak and any juices from the board to the skillet with the peppers and onions and toss for 30 seconds over low heat. Serve with the warm tortillas, chopped cilantro, and lime wedges.