Boil the peeled potatoes in salted water for 15 to 20 minutes, or until they are tender. Then drain them well, mash with salt, white pepper, and butter, and stir in the Gruyère. Keep the mash warm while you prepare the filling.

Combine the fish stock, dry vermouth, onion, fennel, carrot, celery, bay leaf, and saffron in a large pan. Bring it to a boil, then lower the heat and simmer for 5 minutes so the broth takes on flavor.

Add the white fish, smoked haddock, salmon, and prawns to the broth. Poach for about 3 minutes, then lift everything out with a slotted spoon and set it aside.

Next, strain the broth into a clean pan, discard the solids, and boil the liquid until it has reduced by half.

In another pan, melt the butter over low heat, stir in the flour to make a paste, and slowly whisk in the reduced broth until smooth.

Add the milk and parsley, bring the sauce to a boil, then simmer for 10 minutes.

Stir in the cream and season with salt and white pepper.

Heat the oven to 350°F and butter a large casserole dish. Flake the fish, removing any skin and bones, and spread it in the dish.

Pour in about half the sauce, and save the rest for serving.

Then blanch the spinach for about 1 minute, drain it very well, and slice the hard-boiled eggs over the fish.

Add the spinach on top.

Spread the Gruyère mash evenly over the filling.

Mix the ciabatta crumbs and Parmesan together, then scatter them over the top.

Bake for 20 to 25 minutes, or until the top is golden brown and the pie is hot through.

Reheat the reserved sauce and pour it over each serving.