Remove and discard roots, outer leaves, and tops from leeks. Cut off dark leaves of leeks, leaving 6 inches of bulb; reserve dark leaves and bulbs. Remove stalks from fennel bulbs; reserve stalks and bulbs.
Combine dark leaves of leek, fennel stalks, 4 cups water, wine, and fennel seeds in a Dutch oven; bring to a boil. Reduce heat; simmer 25 minutes. Drain through a colander over a large bowl; discard solids.
Slice reserved leek bulbs in half lengthwise. Cut each half crosswise into thin slices. Rinse with cold water; drain. Thinly slice reserved fennel bulbs.
Add oil to pan; heat over medium heat. Add sliced leek and salt; cook 5 minutes, stirring occasionally. Add garlic; cook 2 minutes, stirring frequently. Add broth, sliced fennel, 2 cups water, squash, potato, juice, saffron, chickpeas, and tomatoes; bring to a boil. Partially cover, reduce heat, and simmer 35 minutes. Stir in pepper; sprinkle with parsley.
Place bread on a baking sheet; sprinkle each slice with 2 tablespoons cheese. Broil 1 minute or until cheese melts. Serve with soup.