Wash and peel the asparagus and cut off the woody ends. Bring salted water with some sugar to the boil and cook the asparagus in it for about 12 minutes. Remove and put aside. Bring 200 ml asparagus stock and cream to the boil in a pot.
Add the sauce thickener while stirring, bring to the boil again and season with salt and pepper. Stir in egg yolk, do not let it boil any more. In the meantime, spread 4 flat ovenproof baking dishes (approx. 16 cm Ø) with fat and dust with flour.
Carefully unroll the pizza dough. Divide into 4 pieces and press into the casseroles. Clean, wash and cut the spring onions into pieces. Wash, clean and halve the mushrooms. Cut the asparagus spears in thirds.
Place asparagus, spring onions, mushrooms and a slice of turkey breast in each of the casseroles. Pour the sauce over them and bake in the preheated oven (electric cooker: 200 °C/ gas: level 3) for about 20 minutes.
Cover with baking paper for the last 5 minutes if necessary. Serve garnished with chervil.