2 tablespoons unsalted butter
1 pound leeks, dark greens removed, split, thoroughly washed, and roughly chopped
3 small russet potatoes, peeled and roughly chopped
4 cups vegetable broth
1 teaspoon lemon juice
1/4 cup heavy cream
4 slices white bread, crusts removed and cut into 3/4-inch cubes
1 tablespoon olive oil
1 teaspoon dried thyme
1 tablespoon chopped chives
salt and pepper to taste
Preheat the oven to 400ºF.
Place a large saucepan over medium heat. Add the butter followed by the leeks, potato, salt, and pepper. Cook until the leeks begin to soften, about 5 minutes. Pour in the broth and bring the soup to a boil over high heat. Reduce to a simmer and cook until the vegetables become very tender, about 20 minutes.
While the soup is simmering, place the bread cubes on a baking sheet. Drizzle with olive oil and season with thyme, salt, and pepper. Toss to combine, then spread the bread cubes so that they are in one even layer. Toast the cubes until they become browned and crisp, tossing halfway through, about 7 minutes in total. Set aside to cool.
Working in small batches in a blender or using an immersion blender, puree the soup until smooth. Strain and return to a clean large saucepan over medium heat. Stir in the lemon juice and cream, then season with salt and pepper, if necessary. Ladle the soup into bowls and place a mound of croutons in the middle of each bowl. Sprinkle the soup with chives, then serve.