Finely chop the mushrooms, ham and onion, toast in butter. Season with parsley, thyme, salt, pepper and leave to cool.
In the meantime, cut the chicken fillets into schnitzel, pat dry, pat carefully and season with salt and pepper. Spread the cold mushroom mixture on the chicken fillets, press down a little and fold up.
Roll out the puff pastry, cut into squares and brush with egg white. Wrap the chicken fillets with it and brush with egg yolk. Bake in the preheated oven on the middle rack at 200 ° C for about 20 minutes (no longer!).