Heat the butter in a frying pan on a medium flame. Hold the roulade in the middle and sear both ends. Now place the roulade in the pan and fry it until golden brown on all sides. Put a lid on the pan and cook the roulade at a low temperature (approx. 100°C - 212°F) Sweetbread with mushrooms/white wine sauce, pappardelle and balsamic carrots) for about 1 to 1.5 hours (= core temperature 62-65°C (143-149°F)). Lift the lid and baste the roulade from time to time. A roulade made of loin is delicious when eaten medium-rare.
Remove the roulade from the pan and wrap it in aluminium foil. Let the roulade rest for 5 minutes before cutting. Remove the strings and cut the roulade into small slices using a sharp, non-serrated knife.
Preparing the sauce: heat the butter in a pan, add the onion, garlic, thyme and bay leaf and fry them. Add the red wine and veal stock and reduce to about 175 ml. Before serving, mix the cold butter into the sauce. This gives the sauce a mild flavour and a nice sheen. Season to taste. Cook the green beans al dente. Wash the potatoes and cut them in half. Blanch them, drain, and roast them in the oven.
Before serving: Arrange the beans and potatoes on a plate, place the roulade on top and serve the sauce separately.