Wash and clean the asparagus, removing the tougher ends.
In a frying pan, fry the finely chopped white onion in a little oil. Add the asparagus, a dash of water and cook until soft.
Prepare the béchamel sauce:
Melt the butter in a saucepan, add the flour and whisk until smooth. Slowly pour in the milk, stirring constantly to avoid lumps, until thick and creamy. Season to taste with salt and a pinch of nutmeg.
Blend the asparagus with the béchamel sauce (put some aside for decoration).
Assemble the lasagne by alternating lasagne sheets, béchamel sauce, diced Brie, Prosciutto di San Daniele and a sprinkling of grated Parmesan.
Finish with the asparagus previously set aside and more Parmesan.
Bake in the oven at 180 °C for about 25 minutes until golden brown.
When the lasagne is taken out of the oven, garnish to serve with slices of Prosciutto di San Daniele.