Soak the Römertopf (clay pot) in water for at least 30 minutes. Bring 1 liter of water to a boil, stir in the granulated stock, and let it cool to room temperature. While the water cools, cut the onions and soup greens into small cubes.
Layer the bacon at the bottom of the soaked Römertopf, then add the onions and soup greens. Mix in the tomato paste, 2 tablespoons of the sugar, and the Provence herbs. Create a hollow in the center for the meat. Season the beef all over with salt and pepper, rub the mustard over it. Place the meat on the vegetable bed, sprinkle with the remaining 1 tablespoon sugar. Pour the cooled broth around the sides.
Cover with the lid and place the cold Römertopf in a cold oven. Heat the oven to 175°C (350°F) and braise for 3 hours without opening the door. After 3 hours, check if a fork easily pierces the meat; if tender, remove the pot and let the meat rest for 5 minutes. If not tender, continue cooking until the meat yields easily.
Stir the sauce to combine. If you prefer a smooth sauce without chunks, strain it through a sieve. Optionally stir in cream to taste.
Slice the meat and serve with the sauce. Mashed potatoes are a traditional accompaniment.