If including lobster, rinse under running water, then cut off the head and tail and place to one side for the garnish.
Remove all the cooked lobster meat from tail and claws and break into bite- sized pieces if necessary.
Cut the tiger prawns in half lengthways and remove the black vein.
Dice the salmon, haddock and hake into 4cm cubes.
Heat a saucepan, add the mussels and white wine, cover with a lid and place over a high heat.
Shake the pan every now and then to cook the mussels until they open (around 2 minutes).
Drain and reserve the cooking liquor (you will need this for the sauce).
Allow the mussels to cool for 5 minutes, then remove the meat from the shells, disposing of any unopened mussels.
Melt half the butter in a large saucepan, add the sliced leeks and fennel and cook gently without colouring for 3 minutes, then add the fish stock and bring to a simmer.
Add the salmon, haddock and hake and simmer for a further 2 minutes only.
Gently remove all the fish, leeks and fennel from the pan with a slotted spoon and place in a pie dish.
Add the raw prawns, cooked mussels and lobster meat.
Leave the remaining fish stock simmering in the pan.
Strain the reserved cooking liquor from the mussels through a fine sieve into the fish stock.
To make the sauce, melt the remaining butter in a pan over a medium heat, stir in the flour and cook for a further 2 minutes.
Gradually whisk the hot fish stock into the pan to thicken and cook for a further 5 minutes.
Season to taste, then whisk in the cream and mustard, ensuring there are no lumps.
Remove from the heat and leave to cool for 30 minutes.
Preheat the oven to 180C/ Gas mark 4.
Whilst the sauce is cooling, place the potatoes in a large pan of salted water, bring to the boil and simmer for 15 minutes or until cooked through.
Drain in a colander, then return the potatoes to the pan and mash.
Mix in the milk, egg yolks and butter and season.
Pour the cooled sauce into the pie dish, making sure it just covers the fish completely (keep any remaining sauce to one side, then gently reheat and serve with the pie).
Top with the mashed potato and bake in the oven for 20 minutes, then sprinkle the top with breadcrumbs and cheese and return to the oven for a further 10 minutes until bubbling and golden brown on top.
Garnish with the lobster head and tail for ‘eyes to the sky’.