In a large bowl, combine the chicken thighs, Greek yogurt, garam masala, cumin, curry powder, coriander, 1 teaspoon turmeric, chili powder, chili flakes, and lemon zest. Mix well to coat the chicken evenly. Marinate for at least 30 minutes (or overnight for deeper flavor).
In the slow cooker, add chopped onion, shallot, garlic, and ginger.
Pour in the coconut milk, tomato paste, and remaining teaspoon of turmeric. Stir to combine.
Add the marinated chicken along with all the marinade into the slow cooker. Mix gently.
Cover and cook on low heat for 6-7 hours or on high heat for 3-4 hours, until the chicken is tender and cooked through.
Serve hot over rice or with naan bread, garnished with fresh cilantro if desired