In a medium bowl, whisk together the yogurt, lemon juice, ginger, garlic, spices, and salt. Add the chicken and stir to coat. Cover with plastic wrap and refrigerate for at least 30 minutes or overnight.

Heat the ghee in a sauté pan over medium-high heat. Remove the chicken from the marinade and discard the marinade. Add the chicken to the pan and cook for about 3 minutes. Transfer the chicken to a plate and set aside.

In the same sauté pan, heat the ghee over medium heat. Add the onions, garlic, and ginger and cook for 5 minutes until the onions wilt. Stir in the spices, butternut squash, tomatoes, and water. Lower the heat to a simmer and let bubble for 10 minutes until the squash is fork-tender.

Remove from heat, transfer to a blender, and purée until smooth.

Pour the sauce back into the pan, add the chicken, and stir in the cream, sugar, salt, and cayenne if using. Simmer for another 10 minutes until the chicken is fully cooked.

Spoon the chicken over rice and serve garnished with cilantro and with toasted naan on the side.