1. Halve leeks (white and pale-green parts only) lengthwise; slice into 1/4-inch-thick half-moons. Rinse well; drain. In a large Dutch oven (or other 5-quart pot), melt butter over medium heat. Cook leeks until tender but not browned, stirring occasionally, 6 to 8 minutes.

2. Add russet potatoes, reduced-sodium chicken broth, and 2 cups water. Bring to a boil; reduce heat to a simmer, and cook until potatoes are tender, 15 to 20 minutes. Remove from heat.

3. Puree with an immersion blender or in a regular blender (working in batches and not filling blender more than halfway) until smooth. Stir in half-and-half and a pinch of ground nutmeg. Season with coarse salt and ground pepper. Serve warm or chilled, garnished with chopped fresh chives.