Mix all the marinade ingredients together to form a smooth paste. Add your raw chicken breasts to the bowl. Stir to evenly coat the chicken in marinade.Cover and place in the fridge for 3 hours.
Heat an oven to 400F/ 200C.
Line an oven tray with baking paper and place your chicken breasts on the tray. Bake for 15 minutes.
Remove from the oven and slice your chicken into bite sized pieces. Set aside.
Butter Chicken Sauce
Heat a saucepan on the stove: Add olive oil and sliced onion. Fry for 5 - 6 minutes, stirring frequently, until onion has softened.
Stir in garlic, ginger, garama masala and tomatoes. Bring back to a simmer, cover the pot and simmer for 10 minutes.
Remove from the heat and stir in cream. If you want a smooth sauce, then blend the sauce at this point using a stick blender or transferring the sauce to your blender. It will only need a few seconds of blending.
Add your chicken pieces to the sauce and bring back to a simmer. Simmer for an additional 5 - 10 minutes (to make sure chicken is cooked through)