1Add the cubed chicken thighs to a bowl with red chili powder, garam masala, turmeric, salt, black pepper, cayenne, Greek yogurt, lemon juice, ginger paste, and 5 minced garlic cloves. Mix well and let marinate for at least 30 minutes or overnight.
2Heat 2 tablespoons butter and 1 tablespoon olive oil in a large pan over medium-high heat. Cook the marinated chicken until golden. Remove and set aside.
3In the same pan, add 2 tablespoons butter and 1 tablespoon olive oil over medium heat. Add diced onions and cook until soft, about 5 minutes. Add 5 minced garlic cloves, 2 teaspoons ginger paste, cumin, garam masala, red chili powder, ground coriander, salt, and sugar. Stir and cook for 1–2 minutes.
4Add passata (tomato sauce) and cashews if using. Gently simmer for 5 minutes. Stir in the heavy cream.
5Using a handheld blender or transferring to a blender, carefully blend the sauce until smooth. Optionally add hot water to adjust consistency.
6Return the blended sauce to the pan along with the reserved chicken. Keep the heat low and simmer for 10–15 minutes until the chicken is cooked through. Add the remaining 2 tablespoons of butter.
7Finish with a squeeze of lime and lots of fresh cilantro. Serve with basmati rice and warm naan.