In a large bowl, combine the plain yoghurt, garlic, ginger, lemon juice, half of the garam masala, salt, and vegetable oil. Mix well and marinate the chicken thighs for at least 3 hours in the refrigerator.

Heat a non-stick pan over medium-high heat with a drizzle of vegetable oil. Cook the marinated chicken in batches for about 4-5 minutes per side until charred and cooked through. Set aside.

In the same pan, reduce heat to medium, melt 30g of butter or ghee, and sauté the chopped onion until soft and golden. Stir in the remaining garlic and ginger, cook for 2 minutes.

Add turmeric, remaining garam masala, ground coriander, and cumin. Cook for 1 minute before adding the tomato passata and 400ml of water. Bring to a simmer for about 10-12 minutes.

After simmering, transfer the sauce to a blender and blend until smooth. Return to the skillet on low heat.

Stir in cream and another 50g of butter or ghee, mix well, and add the cooked chicken back into the sauce. Simmer gently for 5 minutes.

Serve the Chicken Tikka Masala over basmati rice or with naan. Garnish with fresh cilantro if desired.