Preheat oven to 375 degrees.
Over medium heat in an oven safe skillet or cast iron skillet, melt butter then add diced onion and cook for 2-3 minutes until it softens and begins to brown.
Next stir in flour and continue to cook for 2-3 minutes until the flour browns and a roux begins.
Whisk in chicken stock and heavy cream.
Season with salt and pepper to taste.
Next, add in mixed vegetables and shredded chicken.
Turn heat down to medium low and cook filling until it has thickened, coating the back of a spoon, and the vegetables and chicken are softened and warmed through.
Turn off heat and top the filling with full box of puff pastry.
Brush the top of the puff pastry with egg wash of egg and water.
Bake for 35-40 minutes until puff pastry is golden brown and filling is bubbling beneath then rest before serving.