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

Preheat the grill or skillet over medium-high heat. Cook the marinated chicken for about 5-7 minutes on each side, or until fully cooked. Remove from heat and set aside.

In a large skillet, heat vegetable oil over medium heat. Add chopped onion and sauté until translucent. Add garlic and ginger, cooking for another minute.

Stir in crushed tomatoes, heavy cream, garam masala, cumin, salt, and sugar. Simmer for 10-15 minutes, stirring occasionally.

Add the cooked chicken to the sauce and stir to combine. Cook for an additional 5 minutes. Garnish with fresh cilantro before serving.