How to Make It
In a large pot, melt the butter in 1 tablespoon of the olive oil. Add the onion, cover and cook over low heat, stirring occasionally, until softened, about 5 minutes. Add the broth and water along with the parsnips and bring to a boil over high heat. Cover and simmer over low heat until the parsnips are tender, about 30 minutes.
Meanwhile, in a small skillet, heat the remaining 1 tablespoon of olive oil. Add the prosciutto and cook over moderately high heat, tossing, until crisp, about 30 seconds. Using a slotted spoon, transfer the prosciutto to a paper towel–lined plate to drain.
Working in batches, puree the soup in a blender until very smooth. Transfer the soup to a clean saucepan. Stir in the cream and milk and bring to a simmer over moderate heat, stirring a few times. Season the soup with salt and pepper and ladle into bowls. Garnish with the prosciutto and thyme leaves and serve.