Wash and peel the asparagus and cut off the woody ends. Cut the asparagus into pieces. Cook in 1/2 litre of boiling salted water for about 12-15 minutes, drain and collect the asparagus water.
In the meantime clean, wash and slice the mushrooms. Wash sage and cut finely. Peel and finely chop the onion. Heat the fat. Fry the onion and mushrooms for about 5 minutes, turning them over.
Season with salt, pepper and sage. Whisk 2 tablespoons of cream and egg yolk. Add remaining cream and asparagus water to the mushrooms and bring to the boil. Coarsely chop the gorgonzola and melt in it.
Add the asparagus. Bind with whisked egg yolk, do not let it boil. Season again with salt, pepper and lemon juice.