For the potato ragout with asparagus, cut potatoes into very small cubes (or into very fine fans and then cut crosswise on a vegetable slicer into the finest cubes) and soak in cold water. Fry finely chopped onions in hot oil.
Add drained potato cubes, deglaze with white wine and boil down. Gradually add asparagus stock or soup, so that the potatoes are always covered but do not float. (Pour only as much as is absorbed).
Simmer the potato stew, stirring often, for about 30-40 minutes. Season with salt and white pepper. Cut cooked asparagus into bite-sized pieces, add and heat briefly. Bind with cold pieces of butter, but do not cook any more.