In a ziptop bag or a shallow dish, combine orange juice, lime juice, olive oil, soy sauce, minced garlic, chili powder, ground cumin, paprika, brown sugar, chopped cilantro, and minced jalapeño. Stir well to combine.
Put the steak in the large sealable bag. Press out the air and seal the bag tightly. Massage the marinade into the meat and refrigerate for at least 2 hours, or overnight if time allows. Don’t go over 12 hours as the meat will become mushy rather than tender.
To Grill:
Preheat the grill and prepare it for direct heat cooking over high heat. Clean and lightly oil the grill grates.
Remove the steak from the marinade, shaking off the excess. Pat the steak dry with paper towels, and season with salt and pepper and sugar if using.
Grill the steak for about 6-8 minutes per side for medium-rare, or to your preferred doneness.
To Broil:
Turn on the broiler, and position your oven rack so the steak will be about 4 inches from the heating element.
Remove the steak from the marinade, shaking off the excess. Pat the steak dry with paper towels, and season with salt and pepper and sugar if using.
Broil the steak for about 5 minutes per side, or to your desired level of doneness.
Stovetop:
Before marinating, if your steak is too large to fit your skillet, slice it in half.
Once ready to cook, remove the steak from the marinade, shaking off the excess. Pat the steak dry with paper towels, and season with salt and pepper, and sugar if using.
Heat a cast-iron skillet over high heat. Add a bit of oil to the pan - enough to coat the bottom lightly. Once the oil starts to shimmer or smoke slightly, carefully place the steak in the skillet.
Sear the steak for about 3-5 minutes on each side for medium-rare, or until your desired level of doneness. To avoid overcrowding the pan and ensure even cooking, cook each piece of steak separately if you had to cut it in half.
To Serve:
Allow the steak to rest on a sheet pan fitted with a wire rack for at least 5-10 minutes, tented with foil to keep it warm.
Slice the steak thinly against the grain, then serve on warm tortillas with fresh cilantro, diced onion, sliced jalapeno, and a squeeze of lime.
Video
Notes
Don't over marinate the steak. Anything more than 12 hours is too long to marinate flank steak. The meat will start to break down and become soft and mush. Something between 2 and 12 hours is ideal.
Drying the steak is important. Too much moisture still in and on the steak will cause a steamy situation in your grill or oven that isn't ideal. Use paper towels to soak up the liquid and remove bits of jalapeno and garlic that might burn.
You can also cook marinated flank steak on the stovetop. I suggest using a cast iron pan that you've preheated. Cook the steak for 3-5 minutes per side until you're happy with the doneness.
Resting is an important step that will allow the juices inside the cooked steak to redistribute. If you slice into it too soon, all of those juices will flow right out!
Storage: Keep leftovers in an airtight container in the fridge for up to 3 days.
Steak Doneness: Use a thermometer to check the internal temperature of the meat according to this list: Rare: 125°F/52°C, Medium rare: 135°F/57°C, Medium: 145°F/63°C, Medium well: 155°F/68°C.