1) Place cod fillets in cold water for about 3 hours.
2) Preheat oven to 400° F.
3) Take a saucepan and simmer potatoes and cod fillets in it for about 20 minutes. Drain.
4) Mash nicely using a potato masher.
5) Take a skillet and heat butter in it. Add onion slices and sauté until transparent.
6) Take a greased casserole and combine in it onion slices, cod mixture, lemon juice, salt, pepper and mustard.
7) Bake for about 25 minutes.