Combine all the marinade ingredients and pour into a sealable bag or glass dish. Place the skirt steak in the bag or dish and let marinate overnight
Turn grill on to high
Once grill is preheated, place the tomatillos on well-oiled grates. Turn frequently for roughly 10 minutes, until tomatillos are slightly charred on the outside
Before the tomatillos are finished roasting, add the cut onion and jalapeƱo (if desired) for a few minutes, until outsides are slightly charred
Remove the vegetables and place the steak on the grill, cooking for roughly 2-3 minutes per side, depending on thickness
Once the steak is cooked, let rest and make salsa. Place the charred vegetables, cilantro, and cumin into a food processor and pulse until combined yet chunky. Remove from the food processor and add the diced watermelons