Marinate the Chicken: In a large bowl, combine full-fat Greek yogurt, garam masala, ground cumin, coriander, curry powder, turmeric, chili powder, and lemon zest. Toss in the boneless chicken thighs to coat well, cover, and marinate for at least 30 minutes—or overnight for deeper flavor.

Brown the Chicken: Heat a tablespoon of olive oil in a skillet over medium heat. Brown the marinated chicken in batches for 2-3 minutes on each side until golden brown. Once done, set the chicken aside on a plate.

Sauté Aromatics: In the same skillet or the slow cooker, add a bit more oil if necessary. Sauté chopped onions, shallots, garlic, and ginger until they become golden and fragrant, about 10 minutes.

Build the Sauce: Stir in the marinade that remains in the bowl, along with tomato paste and full-fat coconut milk. Mix until well combined, then let simmer gently for a few minutes.

Slow Cook: Add the browned chicken to the slow cooker, cover, and cook on low for 6 hours or on high for 3 hours, until the chicken is fork-tender and flavors meld beautifully.

Serve: Once cooked, taste and adjust seasoning if necessary. Garnish with fresh chopped coriander or sliced almonds. Serve alongside fluffy basmati rice or warm naan to soak up the delicious sauce.