Knead the butter with the flour and wrap it in cling film and put it in the freezer.
Remove fat and tendons from the veal fillet, cut off the thin end and dice. Sear the fillet on all sides in hot clarified butter for 4-6 minutes, then remove from the pan.
Roll the meat in the rosemary. Place the thinly sliced bacon side by side so that the meat can be completely covered. Tie everything up into a package with cooking twine. Once the meat has cooled down, wrap it tightly in aluminum foil and let it rest in the refrigerator.
For the sauce, roughly dice the vegetables and sauté in the frying fat. Roast with tomato paste and deglaze with red wine. Add the veal stock and reduce to a third. Then pass everything through a sieve and set aside.
Take the meat out of the refrigerator one hour before processing. Preheat the oven to 80 degrees top / bottom heat and place a bowl under the wire rack. Place the meat on the grid over the dish and cook for about 1.5 hours. Check the core temperature with a meat thermometer. At 57-60 degrees it is pink and tender and the bacon is crispy.
Heat the sauce on the stove. Season to taste with salt and pepper and, if necessary, thicken with a sauce thickener.
Remove the cooking twine from the meat, cut into slices and serve with the sauce.