Put cilantro, jalapeño syrup, olive oil, soy sauce, fish sauce, garlic, chipotle chiles, cumin, and coriander in the bowl of a blender. Purée until very smooth. Taste and add salt and pepper if desired; transfer ⅓ cup marinade to a covered container and refrigerate.

Transfer remaining marinade to a non-reactive container and add the beef or pork; turn to coat. Cover and refrigerate, stirring occasionally, for at least 3 hours, or up to overnight. Remove from the refrigerator about 30 minutes before cooking to come to room temperature; discard marinade.

Prepare a grill for direct heat grilling. Thread the meat, alternating with onion wedges, onto skewers. Grill, turning and basting with the reserved marinade, until done to your desired doneness, 8-10 minutes for medium-rare beef, 10-12 for pork to reach 145F. Transfer to a cutting board, tent loosely, and set aside for 10 minutes; serve hot.