Skin and bone the fish, and cut into small pieces. Set aside.
Grease the bottom and sides of a 9-inch loaf pan heavily with butter. Sprinkle with breadcrumbs, tipping the pan to evenly distribute crumbs.
Also grease a piece of aluminum foil big enough to cover the loaf pan.
Begin heating a saucepan of water to a boil, to use later when baking the pudding.
Using a blender or food processor, purée fish and cream in small batches until smooth.
Place puréed fish in a large mixing bowl. Add salt, nutmeg and cornstarch; beat at medium-high speed until light and fluffy.
Pour mixture into prepared loaf pan. Pick up the pan and bang it on the counter several times, to remove any air bubbles in the pudding.
Seal greased foil tightly over the top of the pan. Place pan in a deep baking pan; add boiling water 3/4 of the way up the sides of the loaf pan.
Bake 60 to 75 minutes, until a tester inserted in the center comes out clean.
Note that during baking, the water around the mold should simmer (but not boil!) constantly; adjust the temperature as necessary.
Let pudding cool 5 minutes.
Pour off any excess liquid; run a sharp knife around the edges of the pan to loosen the pudding. Hold a serving platter on top of the pan and quickly invert the pan and platter to remove the pudding. Soak up any extra liquid with paper towels.