In a mixing bowl, combine chicken, yogurt, lemon juice, garam masala, cumin, paprika, salt, and black pepper. Mix well and marinate for at least 30 minutes.

Heat a grill pan over medium-high heat. Cook the marinated chicken for about 8-10 minutes, turning occasionally until fully cooked. Remove from heat and set aside.

In a saucepan, heat vegetable oil over medium heat. Add chopped onion and sauté until golden brown.

Add minced garlic and grated ginger, and cook for another minute.

Stir in coriander, cumin, turmeric, and chili powder. Cook for 1-2 minutes until fragrant.

Add crushed tomatoes and sugar, and simmer for 10 minutes.

Stir in coconut milk and add the grilled chicken. Simmer for another 5-10 minutes.

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