Pre-heat the oven to 200°C / fan 180°C / gas mark 6. Cut the salmon and cod into large chunks and remove the tough little muscle that sometimes adheres to one side of the scallop (if using). If the scallops are huge, cut in half around the middle. Toss the fish with a splash of lemon juice and freshly ground black pepper. Tip into the bottom of an ovenproof dish that will take the fish leaving enough room for a topping of mashed potato. Sprinkle over the defrosted, drained peas.
Next make the white sauce. Melt the butter in a medium heavy saucepan, add the flour and using a heat-proof spatula or wooden spoon stir over the hear for 2-3 minutes. When you have a smooth bubbling paste that is beginning to turn a sandy brown and starts to smell toasty, switch to a wire whisk and begin to add the milk. Leaving the pan on the heat whisk in the milk in 2 or 3 goes, whisking well to beat out the lumps. Make sure it is well blended. Return to the heat and slowly bring to the boil, whisking all the time.
Once thickened, stir in the grated cheddar, Parmesan and Dijon mustard, lemon zest and juice to taste. Taste and season. Pour over the fish and peas to cover and stir gently.
Spoon the warmed mashed potato in dollops all over the surface then use the back of the spoon to spread out the potato and join the dollops together. Brush the surface with melted butter and fluff up the surface with the back of a fork – this looks good and give nice crunchy bits. Bake for about 20 minutes until golden brown and bubbling. Let it rest for 5 minutes before serving with your favourite greens or salad.