When the grill is prepared, scrape the grates clear. Oil the grill grates with a paper towel dipped in vegetable oil. Place tomatillos, onion, jalapeños, and garlic skewer on the grill (reserve the bowl). Cowl and grill till darkish grill marks seem, about 4 minutes. Flip, cowl, and grill till darkish grill marks seem on the second aspect and the tomatillos begin to soften, 4 to six minutes extra. Switch the whole lot to the reserved bowl.