Into a large bowl, juice 1 lemon, then whisk together with the tahini, honey and olive oil until smooth. Transfer half the mixture to a small serving bowl. Season the chicken all over with salt and add to the large bowl; toss to coat.
To grill the chicken, set a grill to medium-high. (If you’d like to cook the chicken in the oven or a skillet, see Tip.) Generously oil the grates, then add the chicken and cook until the bottom is charred, 5 to 8 minutes. Use tongs to turn over the thighs and continue to grill until the chicken is cooked through, 5 to 7 minutes depending on the size of the chicken pieces; turn thighs over again if necessary. Move thighs to a cooler part of the grill if browning too quickly.
Remove chicken from grill and let cool 1 to 2 minutes. Taste the reserved sauce and season as needed with salt, and loosen the consistency by adding half the lemon juice or a tablespoon of water. Squeeze the remaining lemon half over the chicken, drizzle with the sauce or serve on the side, and garnish with the parsley and sumac, if using.
NYTimes