How to Make It
Combine 2 1/2 cups broth, potatoes, onion, celery, parsley, green onion, thyme, and butter in a 5- to 6-quart pan; cover and bring to a boil over high heat, stirring occasionally. Reduce heat and simmer, stirring occasionally, until potatoes mash easily with a fork, 20 to 25 minutes. Remove from heat and let cool to room temperature, about 1 hour.
Meanwhile, thinly slice a fourth of the cucumber; wrap airtight and chill. Coarsely chop remaining.
Working in batches, whirl potato mixture and chopped cucumber in a blender until smooth; pour into a large bowl. Stir in 3/4 cup crème fraîche, 1/2 teaspoon salt, and 1/8 teaspoon hot sauce. Cover and chill until cold, at least 1 hour, or up to 1 day.
Thin soup with more broth. If desired, add more salt and hot sauce to taste. Ladle into bowls and garnish with sliced cucumber and more crème fraîche.
Vintage Room, Fess Parker's Wine Country Inn & Spa, Los Olivos, California