In a mixing bowl, combine the full-fat Greek yogurt with garam masala, ground cumin, ground coriander, curry powder, turmeric, chili powder, chili flakes, and lemon zest. Add the boneless skinless chicken thighs, coating them well with the marinade. Cover the bowl and let it marinate in the refrigerator for at least 30 minutes, or overnight for deeper flavors.

Heat a large skillet over medium-high heat and add olive oil. Once hot, add the marinated chicken thighs, browning them for about 2-3 minutes on each side until they develop a golden color. Remove the chicken and set aside.

In the same skillet, add a bit more olive oil if needed, and reduce heat to medium. Add diced yellow onion and shallot, sautéing them for about 10 minutes until soft and golden brown. Incorporate minced garlic and grated fresh ginger, stirring for another minute.

Stir in the reserved marinade, tomato paste, and full-fat coconut milk into the sautéed onions and garlic. Mix everything thoroughly and bring to a gentle simmer.

Transfer the sauce mixture into your slow cooker and nestle the browned chicken thighs into the sauce. Stir gently to coat the chicken completely. Cover and cook on low for 6 hours or on high for 3 hours.

Once cooking is complete, check that the chicken is tender. Serve your delightful korma garnished with fresh coriander and sliced almonds.