In a large bowl, combine the yogurt, lemon juice, 1 teaspoon of garam masala, turmeric, cumin, chili powder, half the minced garlic, and half the grated ginger. Mix well and then add the chicken pieces. Toss everything together until the chicken is evenly coated.

Cover the bowl with plastic wrap and refrigerate for at least 1 hour, or overnight for better flavor.

In a large skillet, heat 1 tablespoon of butter over medium-high heat. Add the marinated chicken in a single layer. Cook for 6-7 minutes, turning occasionally, until the chicken is browned and cooked through, reaching an internal temperature of 165°F. Remove the chicken and set aside.

In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Add the diced onion and cook for about 4-5 minutes until softened and golden.

Add the remaining garlic and ginger to the skillet, cooking for about 30 seconds until fragrant.

Stir in the crushed tomatoes, the remaining 1 teaspoon of garam masala, and sugar. Let the mixture simmer for 10 minutes, stirring occasionally until it thickens slightly.

Using an immersion blender, carefully blend the sauce until it is smooth. If you prefer a rustic texture, you can skip this step and leave the sauce as is.

Stir in the heavy cream and return the cooked chicken to the skillet. Simmer for an additional 8-10 minutes, or until the sauce is thick and coats the back of a spoon.

Season the sauce with salt to taste, adjusting based on your preference.

Garnish the dish with fresh cilantro before serving. Enjoy hot over cooked basmati rice for a complete meal.