Melt the butter in a Dutch oven over medium-high heat. Add fennel bulb and leek; sauté 4 minutes. Add potato, water, salt, fennel seeds, pepper, and broth, and bring to a boil. Cover, reduce heat, and simmer 20 minutes or until potato is tender. Place half of the soup in a blender; process until smooth. Pour pureed soup into a bowl. Repeat procedure with remaining soup. Return pureed soup to pan; simmer 5 minutes or until slightly thick. Garnish with fennel fronds, if desired.