1. Peel cucumbers, trim ends, and cut in half lengthwise. Scrape out centers and coarsely chop. Crush garlic and 1/4 teaspoon salt into a paste with flat side of a chef's knife. Combine cucumber, scallion, garlic, 1 1/4 teaspoons salt, buttermilk, and creme fraiche in a blender and process until completely smooth. Add half of each herb and all of cayenne. Pulse to combine.
2. Transfer soup to a bowl, cover, and refrigerate until thoroughly chilled, about 1 hour. Season with salt and pepper, to taste, and lemon, if desired. Divide among 4 soup bowls; garnish with sliced cucumber, scallion and the remaining herbs.