For the Soup
Purée peaches in food processor or blender, adding enough milk to purée the peaches until smooth. Add the remaining milk, the yogurt, sparkling wine, paprika and cayenne, and blend until smooth. Taste and adjust seasoning. Transfer soup to bowl and refrigerate 1 hour.
For the Relish
Combine the peaches, shallot, vinegar, oil and sage in a mixing bowl. Taste and adjust seasoning.
Place a spoonful of relish in the center of a chilled bowl and ladle the soup into bowl. Garnish with pieces of torn prosciutto and a sage leaf.