Heat a large, heavy pot over medium heat and add the Olive Oil. When shimmering, add the sliced Onion, sliced Leek, and thinly sliced Fennel Bulb along with the Fennel Seeds; sauté until the vegetables are soft but not browned, about 6–8 minutes.
Add chopped Tomato, a strip of Orange Peel, Thyme, Bay Leaf and a few crushed Black Peppercorn; cook, stirring occasionally, until the tomatoes begin to break down, about 5–7 minutes.
Pour in the Water, bring to a gentle simmer, then add the Saffron Threads and a generous pinch of Sea Salt. Reduce the heat and let the broth simmer uncovered for 20–30 minutes to concentrate the flavors.
While the broth simmers, peel and cube the Waxy Potato. Add the potatoes to the simmering broth and cook until tender, about 12–15 minutes.
Gently add the pieces of Assorted Firm Mediterranean White Fish to the hot broth and poach over low heat until the fish is just opaque and flakes easily, 4–6 minutes depending on thickness. Taste and adjust seasoning with more sea salt if needed.
Make the rouille: in a small bowl crush a clove of garlic with a pinch of salt, then whisk together with an Egg Yolk, a pinch of saffron threads, a touch of Espelette Pepper and a squeeze of Lemon Juice. Slowly drizzle in olive oil while whisking steadily until you achieve a thick, mayonnaise-like emulsion; season to taste.
Slice the Baguette, brush lightly with olive oil and toast under a broiler or in a 375°F (190°C) oven until golden. Rub the warm toasts with a halved clove of garlic and spread with the prepared rouille.
To serve, place a few slices of Boiled Potato or spoon some waxy potatoes into each shallow bowl, ladle over the fish and hot saffron broth, and finish with a dollop of rouille and the toasted baguette on the side for dipping.
