How to Make It
In a medium saucepan of boiling salted water, cook the potato cubes until tender, about 15 minutes. Drain, reserving 2 tablespoons of the potato cooking water. Transfer the potato cubes to a medium bowl. Add the reserved potato water and the minced garlic and mash until smooth. Whisk in 1 cup of the olive oil in a thin stream until the rouille is creamy. Season the rouille with salt and cayenne.
Preheat the oven to 350°. In a large soup pot, heat the remaining 1/2 cup of olive oil until shimmering. Add the leeks, onion, celery, fennel bulb and 6 of the halved garlic cloves and cook over high heat, stirring frequently, until the vegetables are softened and lightly browned, about 9 minutes. Add the coarsely ground pepper, ground fennel, cumin, paprika, crumbled saffron and tomato paste and cook until fragrant, about 1 minute. Add the wine to the soup pot and cook until it is nearly evaporated, about 8 minutes.
Add the fish stock and thyme bundle to the pot and bring the soup to a boil. Season with salt and pepper and simmer for 10 minutes. Add the monkfish, cod and tuna and simmer the soup until the fish is cooked through, about 8 minutes. Discard the thyme bundle.
Meanwhile, arrange the baguette slices on a baking sheet. Brush the slices all over with olive oil and sprinkle on one side with the oregano. Toast the baguette slices for about 5 minutes, or until the toasts are crisp and lightly golden. Rub the toasts with the 2 remaining halved garlic cloves.
Ladle the fish soup into deep bowls and stir some of the rouille into each. Serve the garlicky croutons, the Parmesan cheese and the remaining rouille on the side.