Heat some olive oil in a saucepan. Add the onions, leeks, celery, potatoes and fennel seeds to the pan. Cook for approx 8 minutes until softened and then add the fennel, bay leaves, white wine and water. Season with a pinch of salt and a good grinding of black pepper.
Bring to a simmer and leave to cook until the potatoes are nearly tender.
Now add the fish and stir gently into the pan. Cook for a further 2 -3 minutes before removing from the heat, ready to serve.
While the soup cooks, to make the garlic mayonnaise, whisk together the egg yolks, mustard and a tablespoon of lemon juice.
Add the sunflower oil in a slow but steady drizzle whisking all the time, then do the same with the olive oil. Season with a pinch of salt. Taste and add a little more lemon juice if necessary. Finish by mixing in the finely chopped garlic.
Chop the parsley and mix some through the soup reserving a little for garnish.
Spoon some mayonnaise on to toasted bread and serve alongside the soup like giant croutons. Sprinkle with any leftover parsley.