Prepare the rouille (a saffron-garlic aioli) first, as it needs time to meld. In a mortar and pestle, or a small food processor, combine the 2 minced garlic cloves, 1 egg yolk, 1 tsp Dijon mustard, a pinch of saffron threads, and a pinch of cayenne pepper. Grind or process until smooth. Slowly drizzle in 120ml (1/2 cup) of olive oil while continuously grinding or processing to emulsify into a thick aioli. Stir in the flesh from 1 roasted red pepper (about 50g / 2 oz), finely chopped or pureed. Season with salt to taste. Cover and refrigerate until ready to serve. This can be made a day in advance.

Toast the baguette slices. Arrange the sliced baguette on a baking sheet. Toast in a preheated oven at 180°C (350°F) for about 8-10 minutes, or until golden brown and crisp. Alternatively, toast them in a toaster. Set aside.

Sauté the aromatics. Heat 120ml (1/2 cup) of olive oil in a large, heavy-bottomed pot or Dutch oven over medium heat. Add the sliced yellow onions, leeks, and fennel. Cook, stirring occasionally, until softened and translucent, about 15-20 minutes. Do not let them brown.

Add garlic and cook briefly. Add the minced garlic (6 cloves) to the pot and cook for another 1-2 minutes until fragrant, being careful not to burn it. If the vegetables seem dry, add a little more olive oil.

Incorporate tomatoes and wine. Stir in the crushed canned tomatoes and tomato paste. Cook for 5 minutes, stirring frequently, to deepen the tomato flavor. Pour in the dry white wine and bring to a simmer, scraping up any browned bits from the bottom of the pot. Let it simmer and reduce by about half, approximately 5-7 minutes.

Add liquids and seasonings. Pour in the fish stock or clam juice. Add the saffron threads, bay leaves, thyme sprigs, orange zest, and red pepper flakes. Season generously with salt and freshly ground black pepper. Stir everything to combine.

Simmer the broth. Bring the mixture to a gentle boil, then reduce the heat to low, cover, and let it simmer for at least 30 minutes, or up to 1 hour, to allow the flavors to meld. Stir occasionally.

Strain the broth (optional, for a smoother soup). For a smoother texture, you can strain the broth through a fine-mesh sieve into a clean pot, pressing down on the solids to extract as much liquid as possible. Discard the solids. Return the strained broth to a simmer. If you prefer a more rustic soup, you can skip this step.

Cook the fish and shellfish. Increase the heat of the broth to a medium simmer. Add the firm white fish fillets and sea bass fillets to the pot. Cook for about 3-4 minutes, or until they begin to firm up. Gently add the mussels, shrimp, and scallops. Cover the pot and cook for another 5-7 minutes, or until the mussels have opened, the shrimp are pink and opaque, and the scallops are cooked through. Discard any mussels that do not open.

Adjust seasoning and serve. Taste the broth and adjust seasoning with salt and pepper as needed. Ladle the bouillabaisse into large, shallow bowls, ensuring each serving gets a generous portion of the various seafood and broth. Garnish with reserved fennel fronds, if desired.

Serve with rouille and toasted baguette. Serve the bouillabaisse with the prepared rouille on the side and the toasted baguette slices. Instruct guests to spread the rouille on the toasted bread and float it in the soup, or to eat it alongside the stew.