The sour rye crumb here makes a great contrast to a creamy, rich soup. Slather extra sage butter on a slice of bread. Verjus, made from the juice of unripened wine grapes, is acidic but gentler than vinegar and helps balance the richness of this soup. Look for it online or in gourmet grocery stores.
How to Make It
Preheat oven to 300°F. Spread bread pieces in a single layer on a rimmed baking sheet. Bake in preheated oven until dry and crispy, 20 to 30 minutes.
Bring 3 cups water and 2 3/4 teaspoons salt to a boil in a large saucepan over high. Add potatoes and leeks, and return to a simmer. Reduce heat to medium-low; cover and simmer until tender, about 30 minutes/
While soup simmers, pulse sage in a food processor until finely chopped, about 10 times. Add butter, verjus, and remaining 3/4 teaspoon salt. Process until smooth, about 1 minute, stopping to scrape down sides as needed.
When soup has finished simmering, transfer to a blender. Secure lid on blender, and remove center piece of lid to allow steam to escape. Place a clean kitchen towel over opening. Process until smooth, about 1 minute. Stir in heavy cream, if desired.
Sprinkle bowls of soup evenly with rye crumbs, and top with about 2 teaspoons sage butter each, reserving remaining sage butter for another use.