Marinate the Chicken:

1. Combine Marinade Ingredients: In a large bowl, mix the yogurt, lemon juice, ground cumin, ground coriander, turmeric powder, paprika, garam masala, chili powder, and salt.
2. Marinate Chicken: Add the chicken pieces to the marinade, ensuring they are well-coated. Cover and refrigerate for at least 1 hour, or overnight for best results.

Cook the Chicken:

1. Preheat Oven: Preheat your oven to 400°F (200°C) or heat a grill.
2. Cook Chicken: Remove the chicken from the marinade and place on a baking sheet. Bake for 20-25 minutes or until cooked through. Alternatively, grill the chicken until charred and cooked through. Set aside.

Prepare the Sauce:

1. Sauté Aromatics: Heat vegetable oil or butter in a large skillet over medium heat. Add the chopped onion and cook until softened and golden brown, about 7 minutes. Stir in the minced garlic and ginger and cook for another 2 minutes.
2. Add Tomatoes and Spices: Add the diced tomatoes and tomato paste to the skillet. Cook until the tomatoes break down and the mixture thickens, about 10 minutes. Stir in ground cumin, ground coriander, paprika, garam masala, and chili powder. Cook for another 5 minutes.
3. Finish Sauce: Stir in the heavy cream and sugar (if using). Bring to a simmer and cook for 5 minutes, allowing the sauce to thicken. Season with salt to taste.

Combine Chicken and Sauce:

1. Add Chicken to Sauce: Add the cooked chicken pieces to the sauce. Stir well to coat the chicken in the sauce and simmer for 5-10 minutes, until the chicken is heated through and the flavors meld.

Serve:

1. Garnish: Garnish with chopped fresh cilantro.
2. Accompaniments: Serve hot with basmati rice or naan bread.