Stuff the meat
Remove the meat from the fridge approx. 1 hr. before cooking.
Make approx. 4 lengthwise incisions in the meat so that it does not fall apart and brush the incisions with mustard. Stuff the incisions with the Gruyère and herbs, season the meat.
Place the lengths of twine on a board, lay the meat on top and tie up with the twine.
Heat a little oil in a cooking pot. Brown the meat all over for approx. 5 mins., remove, sauté the onion, garlic, carrot and celeriac, add the tomato puree and sauté a little longer. Pour in the wine and reduce completely. Add the sugar and bay leaf, pour in the stock, bring to the boil, reduce the heat. Return the meat to the pot. Cover and braise for approx. 1 1/2 hrs. in the centre of an oven preheated to 180°C.
Remove the meat and vegetables, cover and set aside, pour the cooking liquid into a pan and reduce to approx. half the amount. Carve the meat and serve with the vegetables and sauce.