Heat the butter in a roasting pan over medium heat. Grab the roulade in the middle and sear both sides first. Then put the roulade in the pan until golden brown. Put a lid on the pan and cook the roulade on a low heat source, basting regularly. Cook a roulade made of loin meat in approx. 1 to 1½ (= core temperature of 62-65°C (143-149°F)). Roulades made of loin meat can perfectly be eaten when pink in the middle. Remove the roulade from the pan, wrap it in aluminium foil and let it rest for at least 10 minutes before cutting it. Remove the pieces of string and use a sharp non-serrated knife to cut the roulade along the grain into slices.

Put the veal stock in a pan, add the bay leaf and the peppercorns without water until the whole reduces by 1/3. Grease an oven dish with the butter. Put the potato slices in the dish in an overlapping manner. Scatter the sprigs of rosemary across the dish. Add the egg and the garlic to the cream, mix well and pour over the potato slices. Hide some pieces of mozzarella here and there. Carefully remove the sprigs of rosemary before serving.

Serve the veal roulade slices with the potatoes au gratin and the veal gravy. Goes well with green asparagus. The remaining veal gravy can be put on the table in a sauce boat.