In a large bowl, whisk together Greek yogurt, garam masala, smoked paprika, ground cumin, minced garlic, minced ginger, and salt to create a flavorful marinade. Add the boneless chicken thighs, ensuring they are evenly coated with the mixture. Cover and refrigerate for at least 1 hour, preferably overnight.

Heat oil in a heavy-bottomed skillet over medium-high heat. Add the marinated chicken thighs and sear for 5-7 minutes until browned on all sides. Remove chicken from the skillet and set aside.

In the same skillet, lower the heat and add diced onions. Sauté for 5-7 minutes until golden brown. Add minced garlic, ginger, ground cumin, and smoked paprika; cook for an additional minute until fragrant. Stir in the tomato purée and let it bubble for about 5 minutes.

Carefully transfer the sauce to a blender and blend until smooth. Return the sauce to the skillet and stir in heavy cream. Simmer gently over low heat for 5-10 minutes until heated through.

Add the seared chicken back into the creamy sauce, cover and simmer on low heat for approximately 15 minutes.

Garnish with freshly chopped cilantro and serve hot over basmati rice or with naan bread.