Make the Marinade:

If marinating in a zip top bag, make the marinade directly in the bag. Alternatively, make the marinade in a 9×13-inch baking dish.
Place 1/3 cup vegetable oil, 1 3/4 teaspoons kosher salt, 1 teaspoon ground cumin, and 1/2 teaspoon black pepper in the bag or baking dish.
Juice 1 to 2 medium oranges until you have 1/3 cup. Juice 2 medium limes until you have 3 tablespoons.
Finely chop 4 garlic cloves.
Trim and finely chop 1/2 medium jalapeño (about 3 tablespoons; remove the ribs and membrane first if you want a less spicy heat).
Finely chop the leaves and tender stems of 1/3 medium bunch fresh cilantro sprigs until you have 1/4 cup.
Whisk to combine.
Marinate the Steak:

If using flank steak, cut 1 1/2 to 2 pounds in half lengthwise (with the grain), then cut crosswise into 3 to 4-inch long pieces. If using skirt steak, cut 1 1/2 to 2 pounds into 3 to 4-inch long pieces.
Add to the bag and massage until the steak is evenly coated. Alternatively, place in a single layer in the baking dish and flip to coat. Cover the baking dish or seal the bag. Marinate in the refrigerator for at least 2 or up to 8 hours.

Let the steak sit at room temperature while the grill heats. Heat an outdoor grill for direct, high heat (about 500ºF).
Remove the steak from the marinade and pat dry with paper towels (don’t worry too much about wiping off any bits of jalapeño or cilantro). Scrape the grill grates clean if needed. Oil the grill grates with a paper towel dipped in vegetable oil.
Place the steak on the grill grates. Cover and grill until dark grill marks form on the bottom, about 3 minutes. Flip the steak, cover, and grill until an instant-read thermometer inserted into the center of the thickest piece registers 125ºF for medium rare or 135ºF for medium, 2 to 4 minutes more.

Let the steak sit at room temperature while the grill heats. Arrange a rack 5 to 6 inches from the broiling element and heat the broiler to high. Line a rimmed baking sheet with aluminum foil.
Remove the steak from the marinade and pat dry with paper towels (don’t worry too much about wiping off any bits of jalapeño or cilantro). Place on the baking sheet.
Broil until the top is starting to char in spots and an instant-read thermometer inserted into the center of the thickest piece registers 125ºF for medium rare or 135ºF for medium, 8 to 10 minutes.

Heat a large skillet or grill pan over medium-high heat. Meanwhile, remove the steak from the marinade and pat dry with paper towels (don’t worry too much about wiping off any bits of jalapeño or cilantro).
Coat the pan lightly with vegetable oil. Add the steak (cook in 2 batches so as not to crowd the pan). Cook until well browned and an instant-read thermometer inserted into the center of the thickest piece registers 125ºF for medium rare or 135ºF for medium, 3 to 5 minutes per side. (It will be smoky – open a window and turn on the stovetop fan!)
Transfer to a clean cutting board. Let rest for 5 minutes before thinly slicing across the grain.