1. Marinate the chicken by mixing yogurt, lemon juice, ginger-garlic paste, garam masala, turmeric, chili powder, cumin powder, coriander powder, cayenne pepper, and salt in a large bowl. Add chicken and coat well. Refrigerate covered for at least 30 minutes, preferably 2 to 4 hours or overnight.
2. Heat olive oil or ghee in a skillet over medium heat. Add chopped onion and cook until translucent about 5-7 minutes. Stir in garlic and ginger, cooking 1 minute.
3. Add pumpkin puree, diced tomatoes with juice, heavy cream, chicken broth, butter, brown sugar, garam masala, smoked paprika, cinnamon, and nutmeg. Stir to combine.
4. Simmer sauce on low heat for 15-20 minutes, stirring occasionally. Season with salt and pepper.
5. Cook the chicken either by pan-frying in olive oil or ghee over medium-high heat for 5-7 minutes per side until 165°F (74°C) internal temperature, or bake at 400°F (200°C) for 20-25 minutes until cooked through.
6. Add cooked chicken to the sauce and simmer 5-10 minutes to meld flavors. Adjust seasoning.
7. Serve over cooked basmati rice, garnish with cilantro, and serve with naan if desired.