Place 2 lasagne plates in boiling salted water and cook for about 7 minutes. Lift out with a skimmer, rinse and place them side by side on baking paper. Precook the remaining lasagne plates in the same way.
Then keep the pasta water.
In the meantime, wash the asparagus and cut the woody ends off generously. Cut the asparagus into large pieces. Add 1 tsp. sugar to the boiling pasta water, bring to the boil again and cook for about 4 minutes.
Drain and let drain well.
Rinse salmon, dab dry and cut into small pieces. Heat 1 tablespoon of butter in a frying pan. Fry the salmon for 2-3 minutes while turning. Season with salt and pepper.
Peel and finely chop the onion. Heat 3 tablespoons of butter in a saucepan. Sauté onion in it until transparent. Dust with flour and briefly sauté lightly while stirring. Add wine, cream and milk while stirring.
Bring to the boil and simmer for about 5 minutes. Season to taste with salt and pepper. Wash the chervil, shake dry and chop finely. Stir half the chervil into the sauce.
Preheat oven (electric cooker: 200 °C/circulating air: 175 °C/gas: see manufacturer). Grease a fat pan (deep baking tray). Grate the parmesan. Place the lasagne sheets next to each other on the fat pan. Always place asparagus and salmon on one half of each lasagne sheet.
Drizzle some sauce over it. Fold the other half of the lasagne sheets over it.
Spread the rest of the sauce on the lasagne pockets. Sprinkle with parmesan. Bake in a hot oven for about 12 minutes. Serve and sprinkle with remaining chervil.