Make a strudel dough with flour, egg, softened butter, salt and a little lukewarm water. Leave it to rest for thirty minutes wrapped in cling film.
Remove the skin from the onion, cut it into small pieces and sauté it in a saucepan with half of the clarified butter. Add the loosened sauerkraut (rinsed with cold water and squeezed out well, depending on the type and taste) and sauté briefly. Now add the whipping cream and the bay leaf spice and simmer for about ten minutes on low heat. Season to taste with salt and freshly ground pepper.
Portion the fish fillets and fry them in a frying pan with the remaining clarified butter for two to three minutes on each side. Remove from the frying pan, drain on kitchen paper, season with salt.
Preheat the oven to 180 degrees.
Roll out the strudel dough thinly and cut it into squares about twenty cm long (the rest of the dough can be used for other dishes or frozen).
Brush each piece of dough with melted butter and sprinkle with bread crumbs. Put some sauerkraut and a piece of fish on each, brush the fish pieces with a little mustard. Pack into packets, brush them with liquid butter and bake in the heated stove for five minutes until golden brown.
For the sauce, sauté the finely chopped shallot in a frying pan with a little butter. Extinguish with white wine and fish stock and boil by about half. Mix the sauce with the remaining mustard, sa