How to Make It
In a blender or food processor, puree the onion with the lemon juice, salt and pepper. In a large, shallow glass or ceramic dish, coat the chicken thighs with the marinade. Cover with plastic wrap and refrigerate for at least 3 hours and up to 12.
Light a grill. Remove the chicken thighs from the marinade and pat dry with paper towels. Thread the chicken thighs onto four 10-inch metal skewers. Grill over moderately high heat, turning once, until lightly charred and cooked through, about 8 minutes. Lightly dust each skewer with ground sumac. Serve right away, with tahini for dipping alongside.
Sumac, a tangy Middle Eastern spice, is available at specialty shops and online at Kalustyan's.