Mix Marinade: In a large bowl, whisk together orange juice, lime juice, cilantro, minced garlic, cumin, salt, pepper, and jalapeño (if using). While whisking, slowly pour in the olive oil to emulsify the marinade.
Marinate Meat: Place the meat (about 2 pounds of flank or skirt steak) in a resealable plastic bag or shallow dish. Pour the marinade over the meat, making sure it's completely covered. Seal the bag or cover the dish.
Refrigerate: Refrigerate and let marinate for at least 1 hour, ideally for 4 hours or more.
Grill: Preheat the grill to high heat. Remove the meat from the marinade, letting the excess drip off. Grill each side for 3-5 minutes or until it reaches the desired level of doneness.
Rest and Serve: Let the meat rest for a few minutes before slicing against the grain to serve.
Notes
Always marinate in the refrigerator to keep the meat safe from bacteria.
• If using a zip-top bag for marinating, squeeze out as much air as possible to allow the marinade to fully surround the meat.
• Marinate for at least 1 hour, but preferably overnight to maximize flavor and tenderness.
• Discard the marinade after use; if you want some for basting or serving, make extra and set it aside beforehand.
• Grill over high heat to get a good sear and lock in flavors.