Flatten your already thin sliced meat with a meat hammer.
Season the beef slices with salt and pepper, then spread mustard on top.
Cut pickle, carrot and Onion into sticks with the width of the slices.
Put a thin carrot stick, a thinly sliced pickle, and some onion at the upper end of the meat.
Roll the beef up, starting with the end that has the veggies. Secure the end with tooth picks ... ... or metal picks or kitchen twine.
Heat oil in a lage pan and brown the beef from all sides.
Next add some broth.
Optional but recommended: cut the root celery, kohlrabi and leek into small pieces and add them to the pot - this will add a great taste!
Let cook for 1 to 1.5 hours. You know that it's ready when the beef is tender and almost falls apart if you poke into it.
Remove the beef rolls from the pan, so you can work on the gravy.
If you added the vegetables to the broth, pour the sauce through a sieve and try to squeeze some of it through the mesh. Discard what is left in the sieve and work with the sauce.
Mix cold water and corn starch and add it to the sauce. Maybe you will have to add some more water or broth, too.
Let boil for a few minutes and spice as needed to taste with salt, pepper, and mustard.
Best served with potatoes and if you want to go all German: Add red cabbage (Rotkohl) to the menue.
