In a large mixing bowl, whisk together the tahini, lemon juice, minced garlic, cumin, coriander, salt, pepper, olive oil, and warm water until the emulsion is smooth.
Reserve 1/4 cup of the tahini mixture in a separate small bowl to use as a finishing sauce.
Add the cubed chicken to the large bowl with the remaining marinade and toss until every piece is thoroughly coated.
Cover and refrigerate the chicken for a minimum of 30 minutes, or up to 4 hours for deeper flavor penetration.
If using wooden skewers, soak them in water for 20 minutes to prevent burning.
Thread the marinated chicken pieces onto skewers, ensuring they are packed closely but not overly compressed.
Preheat a grill or heavy-duty grill pan over medium-high heat and lightly grease the surface with oil.
Place skewers on the grill and cook for 5 to 6 minutes per side, or until the internal temperature of the chicken reaches 165°F (74°C).
Remove from heat and let rest for 3 minutes before drizzling with the reserved tahini sauce and serving.