Step 1

For marinade:Combine all ingredients in a zip-top bag or shallow baking dish, massaging the marinade into the chicken. Let stand at room temperature while you prepare the sauce, or marinate in the fridge overnight.

Step 2

Heat oil over medium heat in a large saucepan or dutch oven. Add onions and slowly cook until golden, about 20 minutes, reducing heat if they are getting crispy or browning quickly.

Step 3

Add garlic and cook until fragrant, about 1 minute. Stir in garam masala, paprika, cinnamon and salt; cook 1 minute more. Add tomatoes; cook 2 minutes, then add cream and carefully puree using an immersion blender (or standing blender, but do it in batches or the steam will blow the top off).

Step 4

Return sauce to saucepan and bring to a simmer. Remove chicken from marinade and it to the sauce (discarding remaining marinade). Cover, and simmer over medium-low heat until cooked through, about 12 minutes (remove a couple of pieces to make sure they are no longer pink inside). A gentle simmer is required to gently cook the breasts so they don't become tough, and you don't want to overcook them.

Step 5

Stir in butter, taste and add more salt to taste. Serve sprinkled with cilantro, if desired.