In a large bowl, combine Greek yogurt, lemon juice, ginger-garlic paste, turmeric, 1 teaspoon garam masala, and 1 teaspoon salt. Add chicken thighs and toss to coat, letting it marinate for at least 30 minutes.

Heat 1 tablespoon of oil in a large pot over medium-high heat. Sear chicken pieces in batches until golden brown, about 4-5 minutes per side.

In the same pot, melt butter over medium heat. Add diced onions and cook until soft and golden brown, about 5-7 minutes.

Add ginger-garlic paste and cook for 1 minute until fragrant. Then add Kashmiri chili powder and remaining 1 teaspoon garam masala, stirring for 30 seconds.

Pour in tomato puree and crushed tomatoes, adding sugar and remaining salt. Bring to a gentle simmer and cook uncovered for 15 minutes.

Remove the pot from heat and let it cool slightly. Use an immersion blender to puree the sauce until smooth.

Return the sauce to the pot over low heat. Add seared chicken and any juices, then simmer for 5-7 minutes until the chicken reaches an internal temperature of 165°F.

Turn off the heat and stir in heavy cream and adjust salt as needed. Garnish with fresh cilantro.