Try to get small, slender, crisp cucumbers when they're in season; they have fewer seeds and more flavor. Off-season, use the long European cucumbers. Leaving some of the skin on adds color, texture and flavor to the soup.
How to Make the Ultimate Umami-Bomb Vegetarian Demi-Glace
1 pound unwaxed cucumbers, plus 8 thin cucumber slices, for garnish
1 tablespoon sherry vinegar
2 teaspoons kosher salt
2 cups whole-milk yogurt
1/2 cup heavy cream
2 tablespoons coarsely chopped dill
4 salad burnet sprigs, for garnish
Using a vegetable peeler, peel half of the whole cucumbers; leave the skin on the rest. Halve all of the cucumbers lengthwise; scoop out and discard the seeds. Coarsely chop the cucumbers. In a bowl, toss the cucumbers with the vinegar and salt and let stand for 30 minutes.
Using a slotted spoon, transfer the cucumbers to a blender or food processor, reserving the liquid. Add 2 tablespoons of the cucumber liquid and the yogurt, cream and chopped dill to the cucumbers and blend until smooth. Add 1 to 2 tablespoons of the reserved cucumber liquid to thin the soup if needed. Refrigerate until cold, about 1 hour or up to 1 day. Ladle the soup into bowls, garnish with cucumber slices and salad burnet and serve.