Boil the potatoes in a large pot of salted water for 15-20 minutes until tender. Drain well and mash thoroughly with 25g of butter and a splash of milk. Stir in 50g of the grated cheddar and set aside.
Place the 500ml milk, onion halves, cloves, and bay leaf in a saucepan. Bring to a gentle simmer over medium heat, then immediately remove from heat and allow to infuse for 10 minutes. Strain the milk into a jug and discard the aromatics.
Melt the remaining 50g of butter in a medium saucepan over low heat. Whisk in the flour and cook for 2 minutes to create a roux, ensuring it does not brown.
Gradually whisk the infused milk into the roux, a little at a time, until a smooth sauce forms. Simmer gently for 5 minutes until thickened, then stir in the chopped parsley and nutmeg. Season with salt and white pepper.
Preheat the oven to 200°C (180°C fan/Gas Mark 6).
Scatter the cod, salmon, smoked haddock, and prawns into the bottom of a large ovenproof baking dish. Nestle the hard-boiled egg quarters evenly among the fish.
Pour the warm parsley sauce over the fish and eggs, ensuring everything is well covered.
Spoon the mashed potato over the fish mixture, spreading it to the edges to seal. Use a fork to create ridges on the surface and sprinkle with the remaining 50g of cheddar cheese.
Bake in the center of the oven for 25-30 minutes until the sauce is bubbling at the edges and the potato topping is golden and crisp.