1In a medium saucepan, melt the fat. Add the onion and cook over medium heat until lightly browned, 5 to 7 minutes. Add the sauerkraut, bacon, spice bundle, and stock and bring to a simmer, then cover and cook over very low heat for 45 minutes.
2Meanwhile, season the cod with salt and pepper. Pat the tarragon and chervil onto both sides of the fish.
3Line a large backing sheet with parchment paper. Cut the phyllo sheets crosswise on half to make 24.. stack them and cover with damp cloth. For each strudel, spread 1 phyllo sheet on the work surface. Brush it lightly with clarified butter. Top with 5 more sheets, brushing each with butter.
4Set a portion of cod at one end of the phyllo stack, leaving about ½ inch on the bottom and 1 inch on each side, and roll up tightly, tucking in the ends. Brush the strudel with clarified butter and transfer to prepared backing sheet, seam side down.
5Heat the oven to 450 degrees. After the sauerkraut has cooked for 45 min; stir in and add the potato, Riesling, and vermouth. Simmer, uncovered, for 10 min. add the lemon juice and simmer for few minutes longer, then stir in the cream. Bring to simmer, then remove from the heat and taste foe seasoning. Keep warm.
6Bake the strudels in the center of the oven until golden brown, 12 to 15 minutes. Remove from the oven and let the strudels rest on the baking sheet for 2 to 3 minutes.
7Meanwhile, using a slotted spoon, lift the sauerkraut out of the pan, draining it well and letting the sauce drip back into the pan, and mound in warm soup plates. Cut each strudel in half and set on top of sauerkraut.
8Bring the sauce to simmer and strain into a blender. Process briefly at high speed until frothy. Pour the sauce around the sauerkraut in each plate, and serve.
9For tips and variations see p.109 of “Neue Cuisine: The Elegant Tastes of Vienna: Recipes from Cafe Sabarsky, Wallse, and Blaue Gans”