In a zip-locked bag combine the chicken breast with the yogurt. You can add whatever spices you like - I add salt, black pepper, nutmeg, and crushed garlic.
Let the chicken marinate for at least 2 hours in the refrigerator, but no more than one night.
After the chicken finishes marinating, take it out and scrape all the yogurt (don't wash). Discard the yogurt.
Heat a pan over high heat and sauté the chicken breast until it browns on the outside. As the chicken will continue to cook in the cream, it is not important if it is cooked through at this point. However, browning is significant to the taste.
Take the chicken out of the pan and cut it into slices.
Over low heat melt the butter in a saucepan or pan, and add the onion, if using.
Sauté the onion (if using) until it becomes translucent and omits a slightly sweet odor.
Add the garlic and sauté for another minute.
Add the chicken, heavy cream, and spices.
Cook until the cream gets the desired consistency. To serve on pasta use more cream or cook for less time so you have enough sauce remaining to coat the pasta. If serving by itself, let the sauce evaporate more, to get a more concentrated dish.