If necessary, precook the lasagne sheets for some minutes (follow the instructions written on the package).
Peel the pumpkin, remove the pits and cut it into cubes with a lenght of 0,5 cm. Cut the onion in small pieces and roast it in a sauce pan with olive oil. Throw in the pumpkin cubes, salt, pepper and a sprig of rosemary and let it cook for 5 minutes. Then deglace the pumpkin with white wine and remove it from the stove.
For the bechamel sauce heat the milk in a pan. In the same time let the butter melt in another sauce pan and add the flour, wisk it well with a wisk. Add the hot milk to the flour mixture and wisk until it gets smooth. Add salt, pepper, the bay leaf and the nutmeg and let it cook for 5-7 minutes on low heat until thicken up.
Preheat the oven to 180 degrees Celcius or 356 degrees Fahrenheit.
Start to built the lasagne: start on the bottom with a little bit of bechamel sauce. Then put the lasagna sheets over it and continue with more bechamel sauce and ⅓ of the pumpkin ragout. Sprinkle some grated parmesan over it and repeat this step one more time.
At the end put the slices of bacon, the chopped hazelnuts, grated parmesan and some rosemary on the top of the lasagne.
Put the lasagne in the oven and bake it for 25 minutes.