Prepare the Marinade: In a large bowl, whisk together Greek yogurt, olive oil, lemon juice, minced garlic, oregano, paprika, cumin, black pepper, salt, and red pepper flakes (if using) until smooth.
Marinate the Chicken: Add the chicken thigh cubes to the bowl and toss thoroughly to coat every piece. Cover and refrigerate for at least 4 hours, ideally 8–12 hours, for maximum flavor and tenderness.
Prepare Skewers: If using wooden skewers, soak them in water for at least 30 minutes to prevent burning. When ready to cook, thread the marinated chicken onto skewers, leaving a small gap between pieces for even cooking.
Preheat Grill: Preheat your outdoor grill or indoor grill pan to medium-high heat (400–450°F). Clean the grates and lightly oil them with a paper towel dipped in olive oil.
Grill the Kababs: Place the skewers on the hot grill and cook for 10–15 minutes, turning every 3–4 minutes, until the chicken is cooked through, lightly charred, and reaches an internal temperature of 165°F.
Rest and Serve: Remove the kababs from the grill and let them rest for 5 minutes before serving. This helps lock in the juices. Serve with warm pita, garlic yogurt sauce, and a tomato-onion salad.