In a large bowl, whisk together Greek yogurt, lemon juice, garam masala, cumin, coriander, chili powder, and salt until smooth. Add chicken pieces and toss to coat completely. Cover and refrigerate for at least 15 minutes, or up to 24 hours.

Heat 1 tablespoon of butter in a large skillet over medium-high heat. Once melted and sizzling, add the marinated chicken pieces in a single layer (work in batches if needed). Cook for 4-5 minutes until browned on the outside with golden spots. The chicken doesn't need to be fully cooked at this stage. Remove chicken to a clean plate and set aside.

In the same skillet, melt the remaining 2 tablespoons of butter over medium heat. Add chopped onion and cook for 5-6 minutes, stirring occasionally, until soft and golden brown with translucent edges. Add minced garlic and grated ginger, stirring constantly for 30-45 seconds until aromatic.

Reduce heat to medium-low and add all the spices (garam masala, cumin, paprika, turmeric, and chili powder). Stir constantly for about 1 minute to bloom the spices and release their essential oils. The mixture will become very aromatic and the spices will darken slightly.

Pour in the tomato puree and stir to combine with the spiced onion mixture. Season with salt, pepper, and sugar if using. Let the sauce simmer uncovered for 10 minutes, stirring occasionally, until it thickens slightly and reduces by about one-third. The sauce should coat the back of a spoon.

Lower the heat to low and slowly stir in the heavy cream. Return the browned chicken and any accumulated juices to the skillet. Gently simmer for 10-12 minutes until the chicken is fully cooked through (internal temperature should reach 165°F) and the sauce has thickened to a creamy, coating consistency.

Taste and adjust seasoning with additional salt or chili powder if needed. Remove from heat, let rest for 2-3 minutes, then garnish generously with fresh chopped cilantro and serve immediately with rice or naan.