In a large mixing bowl, combine plain yogurt, lemon juice, garam masala, ground cumin, coriander, turmeric, red chili powder, and salt. Add the boneless chicken thighs, ensuring each piece is thoroughly coated. Cover the bowl with plastic wrap and marinate for at least 1 hour, or overnight in the refrigerator for deeper flavor.

Heat a large pan over medium heat and add 3 tablespoons of butter. Once melted, add the finely chopped onion and stir occasionally for about 8-10 minutes, or until the onions turn a beautiful golden brown.

Stir in the minced garlic and grated ginger into the pan, cooking for an additional 1-2 minutes until it's fragrant.

Add the marinated chicken along with any excess marinade to the pan. Cook over medium heat for about 8-10 minutes, stirring occasionally, until the chicken is no longer pink and starts to brown slightly.

Once the chicken is cooked, stir in the tomato puree. Reduce the heat slightly and let it simmer for 10 minutes, allowing the flavors to meld together beautifully.

Lower the heat further, then gently stir in the heavy cream, mixing until fully incorporated. Allow it to cook for an additional 5-7 minutes while tasting and adjusting seasoning if needed.

Remove the pan from heat and garnish your Indian Butter Chicken generously with fresh cilantro. Serve it hot with steaming basmati rice or warm naan.