First, throw out any mussels and clams with broken or open shells. Heat a large saucepan over a high heat and, once smoking, add the mussels, clams and white wine. Cover with a tight-fitting lid and rapidly steam for 3 mins. Once the shells have opened, tip the shellfish into a colander lined with muslin cloth, with a bowl underneath to catch the full- flavoured shellfish stock. Discard any mussels or clams that have not opened.
Heat the oil in the saucepan and add the onion, fennel, carrot and garlic. Cook until softened and browned, about 12 mins, then add the saffron and shellfish stock. Simmer for 5 mins to infuse the saffron. Add the Pernod, tomatoes and 100ml water, and simmer for 5 mins more.
Heat oven to 180C/160C fan/gas 4. Pick the clams and mussels from the shells, reserving some for presentation, and set aside.
To make the mayonnaise, put the mustard, vinegar, egg yolk and garlic in a bowl. Use a stick blender to blitz while slowly drizzling in both oils until thick and glossy. This will take about 5 mins. Chill until needed.
For the croutons, thinly slice the baguette and lay on a large baking tray. Drizzle with a little olive oil and a pinch of flaky sea salt. Bake in the oven for 12-14 mins until golden and crisp.
Once the soup has thickened, transfer to a blender and blitz until very smooth, then pass through a fine sieve. Finish with the brandy, Tabasco and lemon juice. Taste and adjust the seasoning.
To serve, return the soup to the heat to warm through, then drop in the mussels and clams for 1 min. Ladle the soup into two large bowls, drizzle with a little olive oil, and serve the croutons and garlic mayonnaise on the side.