In a medium saucepan, bring the veal stock to a simmer and reduce it to 1/4 of its original volume, until 1 cup of syrupy liquid remains (this may take up to 45 minutes). Set aside.
Prepare the spatzle as directed in the above recipe.
Heat a large heavy skillet over high heat until it is very hot, and add the vegetable oil. Saute the beef strips, tossing frequently, for 2 minutes, or until seared. Add the mushrooms and cook for 3 to 4 minutes more, tossing, until they give up their liquid. Add the mustard, cream, salt, a generous amount of pepper, and the reduced veal stock. Stir to mix thoroughly and remove from the heat.
Make a bed of the spatzle in each of 6 shallow heated bowls, top with a generous spoonful of the ragout and lots of sauce, and sprinkle a few scallions over each.