Boil potatoes in salted water for 15-20 minutes until tender.
Drain and mash potatoes with butter, salt, and white pepper.
Stir in grated gruyère cheese and keep the mixture warm.
Boil fish stock, vermouth, onion, fennel, carrot, celery, bay leaf, and saffron in a pan.
Simmer the broth for five minutes.
Poach white fish, haddock, salmon, and prawns in the broth for three minutes.
Remove fish with a slotted spoon and strain the broth, discarding solids.
Simmer the broth until reduced by half.
Cook butter and flour together to make a paste.
Whisk in the reduced broth and milk until smooth.
Add parsley and simmer the sauce for ten minutes.
Preheat oven to 180°C.
Stir double cream into the sauce and season to taste.
Layer flaked fish and half the sauce in a buttered casserole dish.
Blanch spinach in boiling water for one minute and drain well.
Place sliced eggs and spinach over the fish.
Spread mashed potatoes over the top.
Top with ciabatta crumbs mixed with parmesan cheese.
Bake for 20-25 minutes until golden brown.
Reheat remaining sauce and serve with the pie.