Combine all marinade ingredients except clarified butter in a bowl. Cover and marinate overnight or a minimum of 2 hours.
In a large Dutch oven, add clarified butter and heat on high. Add chicken in batches, ensuring not to crowd the pan, and sear on all sides, creating a deep brown crust. Remove with a slotted spoon and set aside.
To make the sauce: using the same Dutch oven, melt 1/4 cup of butter and add tomato sauce, sugar, red chili powder, 1 tablespoon of fenugreek leaves, and garam masala. Stir, scraping all the bits of spice leftover from frying the chicken in the pan. Continue to cook until tomatoes reduce, thicken, and butter begins to separate (approximately 15-20 minutes).
Preheat oven to 425 degrees. Add remaining butter, heavy cream (reserving a few tablespoons), and salt to taste.
Return chicken to pan, bringing the mixture to a simmer. Cook for 5 minutes, until chicken is cooked through. Garnish with remaining dried fenugreek leaves and a drizzle of heavy cream.
Place naan on a cookie sheet and top with 1/2 cup of butter chicken, making sure to leave a border around the edges. Top with mozzarella cheese, green pepper, and onions.
Bake for 12-15 minutes, until cheese is gooey and has begun to brown. Garnish with coriander, and cut into pieces.