A humble note from the Surreal Gourmet: the payoff from the purple potatoes is somewhat underwhelming. If purple potatoes are easily available, by all means use them. Otherwise, substitute with Yukon gold potatoes.
Cut off the top 1/4 of the pointy end of the garlic bulb so that all of the cloves are exposed. Drizzle with olive oil, wrap in aluminum foil and bake for about 1 hour, or until the top is browned and the individual cloves are soft throughout. Let cool.
Melt the butter in a soup pot over medium-high heat. Add the leek and onion and stir for about 5 minutes, or until they are translucent but not yet brown. Add the wine and cook for 2 more minutes.
Add the potatoes and stock. Bring to a boil, then reduce heat to medium-low and simmer for 40 minutes. (Adjust heat as required to simmer.)
Add the salt, pepper, and sage. Let the soup cool.
Extract the individual garlic cloves (squish them all out by applying pressure to the bottom of the bulb, or remove them individually with an escargot fork). Add them to the stockpot.
If using fresh sage, remove sprigs, then puree the soup in a blender or food processor. Refrigerate for a minimum of 1 hour.
Just before serving, add 1 part milk for every 3 parts soup, and blend in a blender for 15 seconds to aerate. Serve cold in chilled bowls.