Halve the 2 onions and the 4 small pickles for the rouladen, cut them into strips and set aside.
For the sauce, clean the 2 onions, 1 stalk leek, 2 carrots and 0.25 celery root, cut them into rough chunks and set aside separately.
Carefully pound the 4 slices beef for rouladen flat between two oiled sheets of plastic wrap.
Lay the slices out side by side and spread 2 tbsp hot mustard on the top side.
Season the mustard side with salt and pepper and lay 2 slices of bacon on each.
Distribute the onion and pickle strips over the bacon.
Fold in the sides of each roulade and carefully roll up the slices.
Tie each roulade up like a parcel with kitchen twine so they don't come open while searing.
Heat 1 tbsp clarified butter in a Dutch oven and sear the rouladen well on every side.
Remove and set aside.
Heat 1 tbsp clarified butter and sear the prepared diced vegetables well so they develop roasted flavors.
Add 1 tbsp tomato paste and brown it as well.
Deglaze with 0.25 l red wine and let it reduce completely.
Then pour in another 0.25 l red wine and 0.5 l beef stock and briefly bring to a boil.
Carefully place the rouladen and the spices (1 bay leaf, 1 sprig thyme, 2 allspice berries and 4 black peppercorns) in the sauce.
Cover the Dutch oven with a lid and braise the rouladen over low heat for 2 hours.
When the rouladen are done, take them out of the pot and keep them warm.
Pour the sauce through a sieve and reduce it in a pot over the highest heat until creamy.
Then put the rouladen back into the warm sauce.
Carefully remove the kitchen twine from the rouladen and serve and enjoy with the side dishes.