Wash potatoes thoroughly and cook in boiling salted water for about 20 minutes. Clean and wash the asparagus and peel them from head to toe. Cook the asparagus in 1/2 litre salted water for about 20 minutes.
Wash the herbs, dab dry (put some chervil aside for garnishing) and chop finely. Drain the potatoes, remove the skin and keep warm. Remove asparagus from the water and keep warm.
Drain the fat in a saucepan, add the flour, sweat it on and deglaze with the asparagus water while stirring. Add chopped chervil, parsley and the lemon juice to the sauce and stir. Season to taste with salt and pepper.
Arrange asparagus, potatoes, sausage and sauce on a plate. Garnish with remaining chervil leaves and sprinkle with coloured pepper. Garnish as you like with lemon wheels.