Preheat your oven to 400°F.
Heat butter in large skillet over medium heat.
Cook chicken pieces until golden brown all over.
Add onions and garlic to the pan.
Cook until onions become soft and translucent.
Add sliced mushrooms to the chicken mixture.
Sprinkle flour over everything and stir well.
Slowly pour in chicken stock while stirring.
Add cream and season with salt and pepper.
Simmer until sauce thickens nicely.
Roll out puff pastry on floured surface.
Line pie dish with bottom pastry layer.
Pour filling into the pastry-lined dish.
Cover with second sheet of puff pastry.
Seal edges and brush with beaten egg.
Cut small slits for steam to escape.
Bake for 35-40 minutes until golden brown.