Marinate the Chicken: Mix Greek yogurt with garam masala, ground cumin, ground coriander, curry powder, turmeric, chili powder, chili flakes, lemon zest, salt, and pepper. Add chicken thighs, coat well, cover, and refrigerate for at least an hour or overnight.
Sauté Aromatics: In a skillet, heat olive oil and sauté chopped onion and shallot until softened. Add minced garlic and ginger, cooking until fragrant.
Combine Ingredients in Slow Cooker: Transfer marinated chicken and remaining marinade to slow cooker. Add sautéed aromatics, tomato paste, and coconut milk, stirring gently.
Cook Low & Slow: Set slow cooker on low heat for approximately six hours or until chicken is tender.
Final Touches & Serve: Taste test and adjust seasonings. Serve hot over rice or naan, garnished with cilantro or lemon zest.