The saithe should be left in the fridge to defrost for around twelve hours prior to preparation. This done, dab dry with a paper towel and sprinkle with salt.
Peel the parsnip and chop into rounds about 0.5cm thick. Mix with oil, season with salt and pepper, and cook for around 25 minutes at 280°C.
Take the roasted parsnip out of the oven and mix with chopped parsley.
Place the portions of fish on a baking tray lined with baking paper and coat with a little wholegrain mustard. Place dollops of butter on top.
Place the tray with the fish into an oven pre-heated to 70°C and bake for around 25 minutes. If your oven has a needle thermostat, set it to 56°C.
Arrange the roasted parsnip on a plate (stack it in layers if possible) and place the portions of saithe on top.