Chop the onions and garlic and begin heating oil over a medium heat in a large heavy bottom pot.
Start frying the onions and then add the garlic after a minute or two.
Meanwhile, peel the sweet potatoes and very roughly chop. When the onions and garlic begin to get a bit of colour on them add the sweet potatoes, salt, pepper and chicken broth.
Cover and let simmer for 20 minutes or until the sweet potatoes are very soft.
Remove from heat while you puree with a hand-held stick blender or use a regular blender. If using a regular blender, puree in batches and make sure you remove the centre portion of the lid and cover with a tea towel, otherwise you will be scalded with the soup as it explodes all over your kitchen.
As the soup simmers (before blending), place the prosciutto on a cookie sheet covered in parchment paper and bake at 400 degrees F for 10-12 minutes, turning once halfway through. When the prosciutto is crisp, remove from oven and set aside to cool.
After pureeing the soup, add the 1/2 cup of creme fraiche and return to a gentle heat. Stir to fully incorporate the creme fraiche.
Crumble the prosciutto and set it aside until you are ready to serve.
Ladle the soup into a bowl and top with a dollop of creme fraiche and some of the crumbled prosciutto.