This classic combination of leeks, potato, and herbs is flavorful enough to be served warm or chilled. Serve it as a first course for a dinner party with roast chicken or on a picnic with a chicken salad sandwich.
2 tablespoons unsalted butter (1/4 stick)
2 cups finely chopped leeks (pale green and white parts only)
1 teaspoon fresh thyme leaves (from 4 medium thyme sprigs)
2 cups low-sodium vegetable broth
1/3 cupheavy cream
Melt butter in a large saucepan over medium heat. When the foaming subsides, add leeks, season with freshly ground black pepper, and cook until leeks are softened, about 5 minutes.
Add potatoes, salt, and thyme and cook until fragrant, about 2 minutes. Increase heat to high, add vegetable broth and water and bring just to a boil. Reduce heat to low and simmer until potatoes are fork tender, about 10 minutes.
Blend soup in a blender in 2 batches until smooth. Return soup to the saucepan over medium heat, stir in cream, season with additional pepper as needed, and cook until heated through, about 3 minutes.