Salt and pepper the fillet. Fry all over in the heated oil for 10 minutes.
Fry the chopped onions, parsley and mushrooms in the butter. Add the cream and simmer until the mixture has thickened. Season with rosemary, salt and pepper and coat the fillet with the onion, herb and cream mixture.
Roll out the defrosted puff pastry into a rectangle and wrap the fillet with the onion, herb and cream mixture. Brush the surface with egg yolk, the edges with egg white and press down well. Put the whole thing on a baking sheet rinsed with cold water and bake in the preheated oven at 175 ° C for about 35 minutes, it should be golden brown.
For chicken breast fillets, I use four smaller fillets and use them to make four small puff pastry packets instead of one large one. Don`t let the puff pastry get too dark.