In a large bowl, mix the Greek yogurt with garam masala, cumin, coriander, curry powder, 1 teaspoon turmeric, chili powder, chili flakes, and lemon zest. Season the chicken lightly with salt and pepper, then add it to the marinade, turning to coat. Cover and marinate for at least 30 minutes at room temp, or overnight in the fridge.

Heat 1 tablespoon olive oil in a skillet over medium heat, or use the saut? function of your slow cooker. Remove the chicken from the marinade (reserving the marinade) and brown the chicken on all sides in batches, about 2?3 minutes per side. Set aside.

In the same skillet or slow cooker, add more oil if needed. Saut? the onion, shallot, garlic, and ginger for about 10 minutes, until golden and fragrant.

Stir in the reserved marinade, tomato paste, remaining 1 teaspoon turmeric, and coconut milk. Bring the mixture to a gentle simmer, stirring to combine.

Return the browned chicken and any juices to the slow cooker. Stir to coat in the sauce. Cover and cook on low for 6 hours or high for 3 hours, until the chicken is tender and infused with flavor.

Garnish with chopped coriander, toasted almonds, or a swirl of yogurt if desired. Serve with rice or warm naan.