In a mixing bowl, combine chicken cubes, yogurt, garam masala, cumin, coriander, turmeric, salt, and lemon juice. Mix well and marinate for at least 30 minutes, 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 saucepan, heat oil over medium heat. Add chopped onion and sauté until golden brown. Stir in garlic and ginger, cooking for another minute until fragrant.

Add crushed tomatoes, cayenne pepper, and sugar. Simmer for 10 minutes, allowing the sauce to thicken.

Stir in heavy cream and cooked chicken. Simmer for an additional 5 minutes, adjusting seasoning if necessary.

Garnish with fresh cilantro and serve hot with rice or naan.