In a mixing bowl, combine the yogurt, lemon juice, garam masala, cumin, coriander, paprika, salt, black pepper, garlic, and ginger. Add the chicken pieces and mix well. Cover and refrigerate for at least 1 hour, preferably overnight.

Preheat the grill or oven to medium-high heat. Thread the marinated chicken onto skewers and grill for about 10-15 minutes, turning occasionally, until cooked through and slightly charred.

In a skillet, heat the oil over medium heat. Add the chopped onion and sauté until golden brown. Stir in the crushed tomatoes, sugar, and salt. Simmer for about 10 minutes.

Add the heavy cream and cayenne pepper (if using) to the sauce, stirring to combine. Cook for an additional 5 minutes.

Add the grilled chicken to the sauce and stir to coat. Cook for another 5 minutes to heat through. Garnish with fresh cilantro and serve with rice or naan.