Cut the peeled potatoes into 1 inch (2,5cm) thick slices.
Coarsely chop the onion.
Melt the butter in the open pan and fry the onion.
Remove from the cooker and retain any excess butter. Wipe out the base with kitchen paper. Arrange the potatoes in the base of the pan.
Cut the cod into convenient portions for serving.
Place on top potatoes plus the onion and butter.
Pour the lemon juice and liquid over the fish. Add seasoning, and parsley.
Close lid gently and bring to pressure.
Cook on a low heat for 10 minutes. Release steam quickly. Serve immediately.