To make the saffron aioli, smash a clove of garlic, a tiny squeeze of lemon juice, and the saffron (if using) with a small pinch of salt in a pestle and mortar until it turns into a mush. Add a tablespoon of mayonnaise and pound again. Stir in the rest of the mayonnaise. Taste and season with a little more lemon juice, salt and pepper.
Give the mussels and clams a good wash in plenty of clean cold water and scrub any dirty ones lightly with a scrubbing brush, pulling off any beardy bits. If there are any that aren't tightly closed, give them a sharp tap. If they don't close up, throw them away.
Heat a large, wide saucepan or stewing pot and pour in a splash of olive oil. Slice up the rest of the garlic and fry it in the oil until lightly golden. Add the wine, tomatoes and the basil stalks and bring to the boil. Simmer gently for 10 to 15 minutes, until the liquid has reduced a little.
Add all your fish and shellfish in a single layer and season with salt and pepper. Push the fish down into the liquid and put the lid on. Cook gently for about 10 minutes or until all the clams and mussels have opened and the fish fillets and langoustines or prawns are cooked through. (Discard any clams or mussels that don't open.)
Toast the bread on a hot griddle pan and get out the serving bowls. Put a piece of toast in each bowl and ladle the soup over the top, making sure the fish is divided more or less evenly. Top each bowl with some fennel tops, basil leaves, a drizzle of extra-virgin olive oil and a big blob of saffron aioli.