Prepare the Marinade: In a large bowl, combine the Greek yoghurt, 1 teaspoon of turmeric, garam masala, curry powder, cumin, coriander, chilli powder, and chilli flakes. Mix well.
Marinate the Chicken: Add the chicken thighs to the marinade, ensuring each piece is thoroughly coated. Allow to marinate for at least 30 minutes at room temperature, or preferably overnight in the refrigerator.
Brown the Chicken: If your slow cooker has a sear function, use it; otherwise, heat a tablespoon of olive oil in a skillet over medium heat. Remove the chicken from the marinade (do not discard the marinade) and brown on all sides. Set the browned chicken aside.
Sauté the Aromatics: In the same skillet or your slow cooker (if it has a sauté function), add a bit more oil if needed and sauté the onion, shallot, garlic, and ginger for about 10 minutes, or until the onions are golden.
Combine and Cook: Stir the remaining marinade, lemon zest, tomato paste, remaining teaspoon of turmeric and coconut milk into the sautéed aromatics. Bring to a gentle simmer, then return the chicken to the slow cooker. Cover and cook on low for 6 hours or high for 3 hours.