Roast the garlic: Place unpeeled cloves on a sheet of aluminum foil; drizzle with olive oil. Close and seal the foil; roast at 375°F (190°C) for 20-25 minutes. Cool, then squeeze garlic out of skins. Set garlic aside.

Wash sweet potatoes, but do not peel. Place on a parchment-lined baking sheet and roast at 350°F (180°C) for 45 minutes to 1.5 hours, until cooked through completely. (Check by stabbing with a paring knife to see if they are soft all the way through.)

In a large, heavy-bottomed pot over medium heat, melt butter until it foams; be careful not to let it brown. Add diced vegetables, and season with salt and pepper. Cook vegetables over medium heat until they are soft and translucent, about 30 minutes; reduce heat if they begin to brown.

When sweet potatoes are cool enough to handle, remove all skin. Cut flesh into uniform chunks, or mash. Add sweet potato and garlic to pot with vegetables. Add stock, bring to a boil, then reduce heat. Simmer 30-45 minutes, skimming off any foam that rises to the top.

When soup is done, purée in batches in a blender or with a hand blender until completely smooth. (You can reduce thickness by adding a little more stock, if necessary.) Strain through a sieve to remove any chunks.

Allow to cool to room temperature, then refrigerate until completely chilled. Before serving, blend in cold cream. Ladle into chilled bowls, and garnish with a dollop of sour cream. Makes 4 to 6 servings.

Variations

• For a spicier soup, substitute cayenne pepper for the black pepper, and add hot sauce.
• For a sweeter soup, drizzle honey or maple syrup on top as a garnish.