In a large bowl, combine the Greek yogurt, garam masala, ground cumin, ground coriander, curry powder, 1 tsp of turmeric, chili powder, chili flakes, and lemon zest. Add the chicken thighs, season with salt and pepper, and mix to coat. Cover and let it marinate for at least 30 minutes, or refrigerate for a few hours for deeper flavor.

In a skillet over medium heat, heat the olive oil. Add the marinated chicken thighs and brown them on both sides for about 3-4 minutes. This step helps to enhance the flavors but can be skipped if you prefer a quicker method.

In the same skillet, add the chopped onion, shallot, minced garlic, and ginger. Sauté until the onions are softened and translucent, about 5 minutes.

Stir in 3 tbsp of the tomato paste and 1 tsp of turmeric into the aromatic mixture and cook for an additional minute. Then, pour in the coconut milk and return the browned chicken to the skillet, stirring to combine.

Transfer the mixture to the slow cooker and set it on low for 6-7 hours, or on high for 3-4 hours. The longer you cook, the more tender the chicken will become.

Once cooked, taste and adjust the seasoning if needed. Serve the Korma over basmati rice or with warm naan bread for a complete meal.