Rinse the meat with cold water and pat dry. Tap a little gently, possibly cut away the edges and tendons. Season with salt, pepper and the mustard.
Preheat the oven to around 80 to 90 degrees.
Heat some frying fat in a frying pan and fry the meat quickly and vigorously for about a minute, place in a pan suitable for the oven and set aside.
Heat some more frying fat and brown half of the onions in it until golden. Then add the tomato paste on a low flame and toast it through. Gradually deglaze with the red wine, continue to roast a little more until a nice, dark color arises, slowly pour on the bouillon. Let simmer briefly and pour over the meat. Cover with silver foil and let cook in the oven for 2-3 hours.
Dust the remaining onions with flour and fry them in the oil until crispy and then distribute them over the meat after serving.