Gather all ingredients. Preheat the oven to 375 degrees F (190 degrees C).
Melt 2 tablespoons butter in a skillet over medium heat. Stir in onion and garlic, and cook slowly until the onion caramelizes to a dark brown, about 15 minutes.
Meanwhile, combine cream, tomato sauce, remaining butter, salt, cayenne pepper, and garam masala in a saucepan over medium-high heat; bring to a simmer.
Reduce heat to medium-low, cover, and simmer, stirring occasionally, for 30 minutes. Stir in caramelized onions.
While the sauce is simmering, toss chicken with vegetable oil until coated. Season with tandoori masala and spread out onto a baking sheet.
Bake chicken in the preheated oven until no longer pink in the center, about 12 minutes.
Add cooked chicken to the sauce and simmer for 5 minutes before serving.
