Eyvind Hellstrøm, the head chef at Bagatelle Restaurant in Norway, served this gratin as a staff meal once when I happened to be there, doing an interview on him. It was absolutely amazing. There is something real about this kind of gourmet peasant food that makes me doubt whether we really need more fancy food.
Preheat the oven to 350°F (175°C).
Bring water to the boil in a medium pot over medium high heat. Add salt and vinegar. Cut the fish into 1 1/2 inch dice. When the water is boiling, add the fish to the pot, reduce the heat to medium, and let simmer for 10-15 minutes until it begins to fall apart. Strain, allow to cool and remove skin and bones.
Melt the butter in a medium pot over medium heat. Sift in the flour. Stir. Add the milk, little by little, stirring constantly. Make sure to whisk out all the lumps. When you have added all the milk, bring it to the boil while stirring frequently. When the sauce begins to thicken, add the nutmeg, orange zest, thyme, caraway, yellow cheese, a pinch of salt, the eggs and half of the parmesan. Stir well, reduce the heat to low (as it thickens, it burns more easily), and let simmer for 2-3 minutes.
Grease an ovenproof dish with butter or baking fat. Place the fish in the dish. Cover with the sauce. Top with the rest of the parmesan and the bread crumbs. Bake on middle rack in the oven for about 45 minutes, until the surface is golden brown.