First, create a ginger and garlic paste by grating a thumb-sized piece of ginger and 6 cloves of garlic into a small bowl.

Cut the chicken thighs into strips, approximately bite-sized.

In a large bowl with the cut chicken, add turmeric, cumin, coriander, garam masala, chili powder, the juice of one lemon, and Greek yogurt.

Add salt and half of the prepared ginger-garlic paste to the chicken. Mix everything thoroughly to ensure the chicken is well-coated.

Cover the bowl and refrigerate for at least 3 hours, or preferably overnight, to marinate.

Roughly dice the brown onion. If using, slice the green chili lengthwise and remove the seeds and pith.

In a heavy-based pot over medium-high heat, melt half of the butter with olive oil. Add the whole spices: cardamom pods, bay leaves, cloves, and a cinnamon stick. Sauté for a minute to infuse the oil.

Add the diced onion and the ground spices (turmeric, cumin, garam masala, chili powder, coriander) to the pot. Season with salt and cook, stirring frequently, until the onions are soft, about 5-7 minutes.

Add the remaining ginger-garlic paste and cook for another 2-3 minutes. Then, add the sliced green chili and cashews, and stir for a minute.

Pour in the water and crushed tomatoes. Stir well, bring to a simmer, and let it cook for about 20 minutes, stirring occasionally. Add more water if needed.

Remove the whole spices (cinnamon stick and bay leaves). Transfer the sauce to a blender and blend until completely smooth.

Pass the blended sauce through a fine-mesh sieve back into the pot to achieve a silky, smooth texture. Press the solids with a spatula to extract all the liquid.

Preheat your grill (broiler) to high. Spread the marinated chicken in a single layer on a foil-lined baking sheet and grill for 10-15 minutes, or until cooked through and slightly charred.

Return the strained sauce to low heat. Add the cooked chicken to the sauce. Season with sugar and more salt to taste.

Stir in the thickened cream and the rest of the butter until fully incorporated and the sauce is creamy.

Serve the butter chicken hot over a bed of basmati rice and garnish with freshly chopped coriander.