Preheat oven to 375 degrees. In a skillet melt a few tablespoons of butter over medium heat. Stir in garlic and onion and cook slowly about 15 minutes until the onion caramelizes to a dark brown.
In a saucepan, over medium-high melt the remaining butter heat along with the heavy cream, tomato sauce, cayenne pepper, salt, and garam masala. Reduce heat to medium-low and simmer for 30 minutes, stirring occasionally. Then stir in caramelized onions.
In the meantime toss the cubed chicken breast with vegetable oil until coated, and season with tandoori masala.
On a preheated oven bake chicken for 12 minutes until no longer pink in the center. Now, add the chicken to the sauce and simmer for 5 more minutes.
Garnish with cilantro and serve with naan or rice.