Soak the dried cod in water for about 24 hours in the refrigerator, removing it several times to drain the water and replace with fresh cold water. Drain the cod, rinse it, and drain it well.
Use a heavy saucepan or deep skillet. Put the cod in the pan, cover it with boiling water, and simmer it over medium heat for 10-15 minutes. Fish is cooked when it flakes easily with a fork.
Remove from pan carefully and place on hot serving dish. Use the fish stock in preparing mustard horseradish sauce.
3 tablespoons butter
3 tablespoons flour
4 teaspoons dry mustard
2 cups fish stock
1/4 teaspoon salt
1 egg yolk
1 tablespoon cold butter
3 teaspoons fresh grated horseradish
Melt the 3 tablespoons of butter in a medium-sized saucepan. Over low heat, stir in the flour until smooth, then blend in the mustard and stir for two more minutes. Slowly add the fish stock while continuing to stir, then stir in the salt.
Cook over low heat 6-8 minutes, stirring constantly. Remove from heat, and stir in the egg yolk and then the last tablespoon of (cold) butter. Then beat in the horseradish just before serving. – scandanaviancooking.com