In a bowl, combine the Greek yogurt, garam masala, ground cumin, ground coriander, curry powder, 1 teaspoon of turmeric, chili powder, chili flakes, lemon zest, salt, and pepper. Mix well. Add the chicken thighs and coat them with the marinade. Let this sit for at least 30 minutes, or if possible, in the fridge for a few hours or overnight.

If you have time, heat 1 tablespoon of olive oil in a pan over medium heat. Add the marinated chicken in batches and brown it on all sides. This step adds flavor but is optional if you are short on time.

In the same pan, add a bit more olive oil if needed. Sauté the chopped onion, shallot, garlic, and ginger until they soften and smell fragrant, about 5 minutes.

Stir in the remaining turmeric and the tomato paste. Cook for a couple of minutes, then pour in the coconut milk. Stir well to combine everything.

Transfer the chicken and sauce mixture into your slow cooker. Make sure everything is mixed well. Cook on low for 6-8 hours or on high for 4 hours, until the chicken is tender.

When the cooking time is up, check the seasoning. Adjust with salt and pepper if needed, and serve the korma hot over rice or with naan.