In a dutch oven or large soup pot, saute fennel and onion in the olive oil until they begin to soften. Add the garlic and saffron, cooking about 1 more minute.
Add the white wine, deglazing the pan (scraping the bottom of it with the spoon to get up any brown bits). Add the tomatoes, stock, bay leaf, and potatoes.
Bring to a boil and cook until the potatoes are tender. Add the mussels and fish (except for the shrimp) to the simmering pot and cook for about 3 minutes. Add the shrimp and cook for another 2-3 minutes, or until the mussels have completely opened. Discard any unopened shells.
Serve in shallow serving bowls with a dollop of the rouille, garnish with parsley, lemon, and/or the reserved fennel fronds.