Heat the oil in a pan over medium heat and cook the onion and garlic until soft, stirring constantly. Add the carrot, celeriac and celery and cook until soft. Push it to one side and add the minced lamb, leaving it be, without stirring, for a few minutes till it has started to brown. Stir and continue cooking, mixing the vegetables in as you go. Pour in the stock and tomato sauce, mix thoroughly, then leave to simmer on a moderate to low setting for a good 20 minutes till it is thick and rich. In the meantime prepare the béchamel sauce. Warm up the milk in a pan with the studded onion. Melt the butter in a large saucepan over medium-high heat until foaming. Add the flour and cook, stirring, for 1-2 minutes or until mixture bubbles and begins to come away from the side of the pan. Remove from heat. Gradually pour in half the milk, whisking constantly with a balloon whisk, until mixture is smooth. Gradually add the remaining milk, whisking until smooth and combined. Place saucepan over medium-high heat and bring to the boil, stirring constantly with a wooden spoon, for 5 minutes or until sauce thickens and coats the back of the spoon. Remove from heat. Add the parmesan and stir until cheese melts. Season to taste with nutmeg, salt and white pepper. Preheat the oven to 180°C. Take out the lasagne sheets and top evenly with a scant of meat mixture and spread the béchamel sauce all over. Roll up sheet, starting at one of the short ends. Place in greased baking dish and put in freezer for half an hour until it’s easy to cut in 4cm rolls. Place on baking tray and bake in preheated oven for 40 minutes until cheese melts and the edges are bubbling.