Thinly slice the cleaned onion and mushrooms.

Cut the chicken breast into small cubes.

In a skillet, melt the butter and add the mushrooms, onions, and chicken. Cook for 10-15 minutes.

Roll out the puff pastry and place the mushroom mixture in the center.

Mix the egg yolk and milk together, then brush the pastry with the mixture.

Bake for 15 minutes at 300°F (150°C).