1. In a large pot, heat half of the olive oil. Add the onion, cover and cook over low heat, stirring occasionally, until softened. 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.

2. In a small skillet, heat the remaining olive oil. Add the prosciutto and cook over moderately high heat, tossing, until crisp. Transfer the prosciutto to a paper towel-,-lined plate to drain.

3. Puree the soup until very smooth. Stir in the 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.