Pre-heat the oven to gas mark 7/425°F/220°C.
You will need a baking dish about 5 cm deep of 2 litre capacity, well buttered.
Start off by boiling the potatoes, with skin on, in a saucepan with enough salted water.
Once boiled though, strain off the water and cover them with a clean tea towel to absorb the steam.
Meanwhile, heat the stock in a medium saucepan, add the bay leaf and season it well.
Then cut the fish in half, if they are large pieces, add it to the saucepan and poach the fish gently for 5 minutes (it should be slightly undercooked).
Using a draining spoon, remove the fish to a plate and strain the liquid through a sieve into a bowl.
In the same pan you cooked the fish in, melt in the butter, whisk in the flour and cook for 2 minutes.
Then gradually add the strained fish stock little at a time, whisking all the time.
When you have a smooth sauce, turn the heat to its lowest setting and let the sauce gently cook for 5 minutes.
Then whisk in the double cream, followed by the gherkins and chopped parsley.
Give it all a good seasoning and remove it from the heat.
Now to make the rösti, peel the potatoes and, using the coarse side of a grater, grate them into long shreds into a bowl.
Then add the melted butter and lightly toss everything together using 2 forks so that the potatoes get a good coat of butter.
Now remove the skin from the white fish and divide it into chunks, quite large if possible, and combine the fish with the sauce.
Next, add the raw scallops and prawns to the fish mixture then spoon it into the prepared baking dish.
Sprinkle the rösti on top, spreading it out as evenly as possible but not pressing it down too firmly.
Finish it off by scattering the grated cheese all over the surface.
Bake on a high shelf of the oven for 35-40 minutes until golden brown.
If you would like to make the fish pie in advance, then you will have to let the sauce cool down completely before adding the fish, scallops and prawns. After you have spread the topping, cover the dish loosely with clingfilm and refrigerate it until you're ready to bake. But you will have to give it an extra 10 minutes of cooking time.
