The day before, wash the tomatoes and dab them dry. Peel the garlic and press lightly with the edge of a knife. Wash the rosemary, shake dry and chop coarsely. Dab meat dry. Heat 2 tablespoons of oil in a frying pan and fry the meat well all around. Place the meat on the fat pan of the oven and rub vigorously with sea salt and pepper, add tomatoes, garlic and rosemary. Cook in the preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for 40-45 minutes. (It is best to measure with a roast thermometer: the meat should have 58-60° C at the core).
Remove meat from the oven and let it cool completely. Collect the stock and pass it through a sieve into a small pot. Add lemonade, beer and tomato puree and let it boil down to half, season with salt and pepper and let it cool down.
Onions peel, halve and in columns cut. Heat 1 tablespoon of oil in a frying pan, sauté the onions in it for about 3 minutes, sprinkle with sugar and deglaze with vinegar, add the fat and braise lightly for about 2 minutes. Mix mustard and honey.
Cut the baguette sticks lengthwise. Cut the meat into thin slices. Spread the baguette with the mustard mixture and fill with the meat. Spread onions and sauce over it. Add remaining sauce and mustard.