1 Rinse cod thoroughly, taking care to remove the tiny bones.
2 Fill a pot with about 4 cm (approximately 2 inches) of water and bring to a boil. Season fish on both sides with salt and pepper.
3 Coat the bottom of a steamer basket with the oil.
4 Put fish in the basket and set in the pot. Cover and steam the fish over high heat for, 8-10 minutes, turning after 5 minutes. (The fish is done when the center bone separates easily from the flesh and the skin can easily be removed.)
5 Meanwhile, whisk together egg yolks and mustard in a bowl over a pan of hot water until creamy, about 5 minutes.
6 Stir in whipping cream and season with salt and pepper. Place the fish on a plate and serve immediately with mustard sauce. Serve with dill potatoes.