In large pot melt butter. Add leeks and cook over medium heat, stirring occasionally. Add potatoes and season with salt and pepper. Add water and bring to a simmer. Cook until vegetables are tender. Place 3/4 of soup into a blender and puree, return to pot and bring to simmer. Add mustard and cheese. Stir until smooth. Add cream and bring to simmer. Pour into bowl and serve.
Recipe Courtesy of Food Network Kitchens