Start by getting the chicken stock together. If you are making the stock from scratch, check out my chicken stock recipe.
Gently brush or wipe any loose dirt from the mushrooms and cut the mushrooms, stems included, into thin slices.
Finely chop the onion and garlic.
Cut the check breasts long ways into quarters and then slice them diagonally into 3/4 to 1 inch pieces.
Finish up the prep by finely chopping the parsley. I don't mind the stems because I think they add more flavor, but if you have a problem with using the stems, leave them out.
Heat up a heavy-bottomed soup pot and melt the butter.
Add the onion and cook for a minute or two, then add the garlic. I find that by adding the garlic after the onion is cooking, it is less likely to burn. It's important to stir the onion and garlic as it is cooking, too. Cook until the onion is soft and translucent. Add the mushrooms. Mushrooms contain a lot of liquid so you may notice more liquid in the pan as they are cooking than when you started. No big deal. Keep cooking until most of the mushroom liquid evaporates. Season with salt and pepper to your own taste.
Add the chicken stock to the pot and simmer for 10 minutes. Remove from heat.
Time to puree the soup. I like to use a hand blender so I don't have to transfer the soup to a blender or food processor. (Also keeps the cleaning to a minimum.) If you have a hand blender, puree the soup directly in the pot to the consistency you like. If you don't have a hand blender, use what you have: remove half of the soup from the soup pot, transfer to a blender or food processor, and puree that half-portion of the soup. Return the puree back to the soup pot with the remaining half-portion. Now it's time to add the cut up chicken breast pieces to the pot and cook over medium heat. Because the breasts were cut up, they won't take long to cook- maybe 4 to 5 minutes, tops. You don't want the soup to boil, just simmer. So keep your eye on it.
Add the dry sherry, half and half, hot pepper flakes, and fresh parsley, and simmer for a couple of minutes more.
Taste for seasonings, adjust if necessary, and serve.