Marinate the Chicken: In a large bowl, combine all the marinade ingredients. Mix well, ensuring the chicken is evenly coated. Cover and refrigerate for at least 2 hours, or preferably overnight.

Cook the Chicken: Preheat your oven's broiler. Spread the marinated chicken in a single layer on a baking sheet lined with foil. Broil for 8-10 minutes, or until the chicken is cooked through and lightly charred. Alternatively, you can grill the chicken for a similar effect.

Prepare the Sauce: In a large pot or Dutch oven, melt the butter and oil over medium heat. Add the chopped onion and cook until softened and lightly golden, about 5-7 minutes.

Bloom the Spices: Add the ginger-garlic paste, garam masala, cumin powder, turmeric powder, and red chili powder to the pot. Cook for 1-2 minutes, stirring constantly, until fragrant. This step is crucial for tempering the spices.

Add Tomatoes and Simmer: Stir in the crushed tomatoes and tomato paste. Bring to a simmer, then reduce heat to low, cover, and cook for at least 30 minutes, or up to an hour, stirring occasionally. This allows the flavors to meld and the sauce to thicken.

Blend the Sauce (Optional): For an ultra-smooth sauce, use an immersion blender to blend the sauce directly in the pot. Alternatively, you can carefully transfer the sauce to a regular blender and blend until smooth (be cautious when blending hot liquids!).

Finish the Sauce: Stir in the heavy cream, crushed fenugreek leaves, and sugar (if using). Season with salt to taste. Return the cooked chicken to the pot and stir to coat in the sauce.

Simmer and Serve: Simmer for another 5-10 minutes to allow the chicken to absorb the sauce's flavors. Garnish with fresh cilantro leaves and serve hot with naan bread, basmati rice, or both.