Slice garlic cloves in half. Coarsely chop onion. Cut root ends from leeks. Cut leeks in half lengthwise. Discard dark green parts. Fan out and rinse under cold water to remove any grit. Thinly slice. Heat oil in a large wide saucepan set over medium heat. Add garlic and onion. Stir often until onion is slightly soft, 3 minutes. Add leeks. Stir often until soft, 5 minutes.
Meanwhile, peel potatoes. Chop into small pieces. Pour 1/2 cup (125 mL) sherry into pan. Scrape up and stir in any brown bits from pan bottom to add flavour. Add water, potatoes and salt. Increase heat to high. Bring to a boil, covered. Stir often. Reduce heat to medium-low. Simmer, partially covered and stirring occasionally, until potatoes are very tender, 20 to 25 minutes. Stir in cream. Remove from heat.
Purée soup in 3 batches in a food processor. Press through a sieve into a clean saucepan. Reheat over medium heat, then stir in remaining 1/4 cup (50 mL) sherry and generous pinches of pepper. Ladle into warm bowls. Crumble in Stilton, if using. Soup will keep well, covered and refrigerated, up to 5 days or freeze up to 1 month.
Nutrition (per serving)
A splash of sherry gives this velvety soup a sophisticated finish.