Puree the coarsely chopped onion with lemon juice, kosher salt, and ground black pepper in a blender or food processor until smooth.

Coat the skinless, boneless chicken thighs with the marinade in a large, shallow glass or ceramic dish.

Cover the dish with plastic wrap and refrigerate for at least 3 hours, or up to 12 hours, to allow the chicken to marinate.

Light a grill and prepare for moderately high heat.

Remove the chicken thighs from the marinade and pat them dry with paper towels.

Thread the marinated chicken thighs onto four 10-inch metal skewers.

Place the skewers on the grill over moderately high heat, turning once.

Grill the chicken until it is lightly charred and cooked through, approximately 8 minutes.

Lightly dust each skewer with ground sumac.

Serve the Lemony Grilled Chicken Kebabs immediately with tahini for dipping alongside.