Prep Steak: Pat steak dry; trim silver skin. Lightly season both sides with a pinch of salt and pepper.
Make Marinade: Whisk lime juice, orange juice, pineapple juice (if using), soy/tamari, Worcestershire, oil, garlic, cumin, smoked paprika, salt, pepper, brown sugar, baking soda (optional), cilantro, and jalapeño.
Marinate: Add steak to a bag/dish; pour marinade over to coat. Chill 1–8 hours (sweet spot: 2–4 hours). Avoid longer to prevent mushy texture.
Veggies: Toss peppers and onion with oil, salt, and pepper.
Preheat: Heat cast-iron over medium-high until just smoking (3–5 minutes). If grilling, preheat to high and oil grates.
Char Veggies: Add peppers/onions to the dry hot skillet. Cook, tossing occasionally, until edges char and veg stay crisp-tender, 5–7 minutes. Transfer to a plate; tent with foil. (Optional: splash pan with 1 Tbsp tequila or vinegar to glaze and toss with veg.)
Sear Steak: Wipe out moisture; add a thin film of oil. Let excess marinade drip off steak. Sear 3–4 minutes first side; flip and cook 2–3 minutes more to 130–135°F (54–57°C) for medium-rare. Thick flank may need +1 minute.
Rest & Slice: Rest steak 5–10 minutes. Slice very thinly across the grain at a slight angle.
Sizzle Finish (Signature Touch): Return skillet to medium heat. Melt 1 Tbsp butter; add zest of 1 lime (optional). Return sliced steak for 30–45 seconds with the veg to heat through. Squeeze fresh lime over.
Serve: Warm tortillas (dry skillet 20–30 sec/side or foil-wrapped at 300°F/150°C for 10 min). Build fajitas with toppings. Finish with cilantro and another lime squeeze.
