Heat the oven to 200C/180C fan/gas 6. Wrap the potato in foil, place on a baking tray and roast in the oven for 1 hour 15 mins. Wrap the fish in a piece of foil, put on the same tray and continue cooking for a further 10 mins or until the fish is opaque and cooked through.
Meanwhile, cook the peas in boiling water for 3 mins or until tender. Drain.
Once cooked, halve the potato and scoop out the filling. Flake the fish, removing any bones and discarding the skin.
Blitz the peas, potato and salmon in a food processor, adding a splash of milk or knob of butter if you like, to loosen the purée. Serve one portion and divide the rest between small containers and freeze.