Prepare the Marinade: In a large bowl, combine minced garlic, lemon juice, Greek yogurt, olive oil, cumin, coriander, paprika, turmeric, cinnamon, cayenne pepper (if using), salt, and black pepper. Mix until a smooth marinade forms.
Marinate the Chicken: Add the chicken thighs to the marinade, ensuring each piece is thoroughly coated. Cover the bowl and refrigerate for at least 2 hours, or up to 8 hours for deeper flavor.
Preheat the Grill: Heat an outdoor grill or grill pan over medium-high heat. Lightly oil the grates to prevent sticking.
Grill the Chicken: Remove the chicken from the marinade, allowing excess to drip off. Place the thighs on the grill and cook for 6-8 minutes per side, or until the internal temperature reaches 165°F (74°C) and the exterior is nicely charred.
Rest and Serve: Transfer the grilled chicken to a cutting board and let it rest for 5 minutes. Slice into strips and serve warm.