Season the beef schnitzel with salt and pepper on both sides.
Brush one side of each schnitzel with mustard and anchovy paste.
Dip the other side of each schnitzel in flour.
Heat 3 tablespoons oil in a large pan over medium-high heat.
Fry the schnitzel on the floured side for 3-4 minutes until golden. Turn and fry the other side for 3-4 minutes until golden. Remove and set aside.
In the same pan, fry the sliced onion in the remaining fat over medium heat for 4-5 minutes until golden.
Add 500 ml beef stock to the pan and scrape the bottom with a wooden spoon to loosen the browned bits.
Add the capers and return the schnitzel to the pan. Cover and braise over low heat for 25-30 minutes until tender, adding more beef stock if the sauce reduces too much.
Remove the schnitzel and set aside.
Whisk 250 ml sour cream with 1 tablespoon flour until smooth. Pour into the sauce along with 250 ml cream and stir well.
Bring the sauce to a boil over medium heat, stirring constantly. Simmer for 1-2 minutes.
Season the sauce with salt, pepper, mustard, and anchovy paste to taste.
Return the schnitzel to the pan and let it steep over low heat for 5 minutes.
Cook 500 g pasta in plenty of salted water according to the package instructions.