Cut onions in thin stripes.
Peel potatoes and cut them in thin slices.
Melt the butter.
Place half of the potato slices in the bottom of pyrex pot. Add pepper and salt. Fill the pot with altemate layers of fish, onion and potato slices, with salt and pepper on them.
Place the pot in 175 oven for 45 minutes.
Distribute cream on top of fish. Serve after 15 minutes..