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.