In a large saucepan, melt the butter in the olive oil over medium heat.
Add the sliced onion and sweat it for about 4 minutes until it gets soft.
Cut the chicken into bite-sized pieces and add it to the onion.
Add the thyme, salt, and pepper and gently fry for about 5 minutes stirring constantly.
Add the mushrooms and peas and continue to cook until the mushrooms get tender.
Add the cornflour to the chicken and mix well.
In the meantime, mix together the cream and the mushroom soup mix in a small bowl.
Add them to the chicken and combine everything together until you get a gentle boil.
Turn off the heat and transfer it to a deep round baking dish.
Cover them with puff pastry and fold the edges to create a seal.
Brush the surface with the whisked egg yolk and create a couple of slits to let the steam out.
Bake in a preheated 200°c oven until golden brown and piping hot. Should take about 30 minutes. Keep an eye not to burn the surface.