Melt the butter in a medium pot on low heat. Add the onion, garlic, and salt and cook, stirring occasionally until the onion is soft and golden, about 10 minutes.

Add peas, parsley, and stock. Bring to a boil over high heat, then reduce to medium and keep the soup at a moderate boil for 5 minutes. Let the soup cool for a few minutes in the pot, then add the cream. Transfer the soup to a blender and puree until smooth.

Set a fine mesh strainer over a large bowl. Pour the soup through the strainer, stirring and pressing down on the solids to push all the liquid through. Chill in the refrigerator for 1 hour.

Before serving, if you using the prosciutto, heat a small skillet over medium high heat. Tear up the prosciutto slices into bite sized pieces. Add them to the skillet and cook, stirring frequently, until the prosciutto is browned and crispy, about 5 minutes. And, in a small saucepan, heat the creme fraiche over low heat until it's liquid, also about 5 minutes.

Ladle the soup into four bowls. Swirl some of the creme fraiche into each bowl and then top with crispy prosciutto and a little chopped parsley.