1) Marinate the steak
In a bowl or zip-top bag, mix orange juice, lime juice, soy sauce, olive oil, garlic, cumin, chili powder, smoked paprika (if using), salt, and pepper. Add the steak and coat well.
Marinade tip: Use a snug bag or a dish where the steak sits in close contact with the marinade, so it actually does its job.
Marinate at least 30 minutes, or up to 4 hours (refrigerated). If marinating longer than 30 minutes, pull the steak out of the fridge 20 to 30 minutes before cooking so it loses the chill.
Safety note: Discard used marinade (anything that touched raw beef).
2) (Optional) Make the quick charred salsa
Heat a cast iron skillet over medium-high. Add tomatoes, onion, jalapeño, and garlic. Cook, turning occasionally, until you get deep char spots all over, 8 to 12 minutes total.
Blend or finely chop with lime juice and salt. If blending, pulse to your preferred texture. Add a tiny splash of water only if it needs help moving. Taste and adjust. If it tastes flat, it usually needs more salt or lime.
3) Cook the steak (grill or skillet)
Before cooking: Remove steak from the marinade and let excess drip off. Pat it dry with paper towels for a better sear.
Grill: Preheat to high (about 450°F to 500°F). Grill 3 to 5 minutes per side depending on thickness.
Skillet: Heat a cast iron skillet over high until very hot. Add 1 to 2 teaspoons neutral oil (and turn on your vent, it will get smoky). Sear steak 3 to 5 minutes per side.
Broiler option: Set an oven rack about 4 to 6 inches from the heat. Place steak on a foil-lined sheet pan. Broil 3 to 6 minutes per side, watching closely.
Target temp: Pull at 130°F to 135°F for medium-rare, or around 140°F for medium. Resting will raise the final temperature a bit.
4) Rest, then slice the right way
Rest the steak 5 to 10 minutes. Then slice thinly against the grain. If using skirt steak, cut it into a few shorter pieces first so slicing is easier.
5) Warm the tortillas
Warm tortillas in a dry skillet over medium heat, about 20 to 30 seconds per side, until soft and lightly toasted. Keep them warm in a clean towel.
6) Build tacos and eat immediately
Fill tortillas with sliced steak. Top with onion, cilantro, salsa, and any extras you love. Finish with a squeeze of lime and a tiny pinch of salt if needed. Taste as you go. It is highly recommended.
Serving note: Plan on 2 to 3 tacos per person with this amount of steak, depending on how heavy you go on toppings.