Preparation Whisk tamarind purée, fish sauce, brown sugar, red pepper flakes, cumin, and 1 tsp. Diamond Crystal or ½ tsp. Morton kosher salt in a medium bowl until combined and a loose paste forms. Set tamarind glaze aside. Pat wings dry with paper towels, then season all over with remaining 2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt. Place in a large bowl; add half of tamarind glaze and toss to coat. Chill 15 minutes. Set remaining glaze aside. Meanwhile, place onion in a bowl of ice water and set aside. Prepare a grill for medium indirect heat (for a charcoal grill, bank coals on one side of grill; for a gas grill, leave one or two burners off). Arrange wings on grate over indirect heat, making sure not to overcrowd. Brush top of each wing with some reserved glaze. Cover and grill 15 minutes. Uncover, turn wings over, and brush other side with glaze. Re-cover and grill until wings are golden brown and a thermometer inserted into the thickest part of a wing registers 165°F, about 15 minutes. Brush wings with more glaze, then move over direct heat and grill until charred in spots, 1–2 minutes per side. Transfer wings to a platter. Drain onion and place on paper towels; pat dry. Scatter onion over wings and top with cilantro. Serve with lime wedges for squeezing over.