Heat 2 tbsp of oil in a frying pan over a medium-high heat and add your chicken. Season with a pinch of salt and fry for 2-3 mins until the chicken is golden. Remove from the pan.
Melt butter in a frying pan over a medium heat. Add your onion, carrots and celery and cook out for 5 mins until soft but not quite taking on colour. Add your garlic and cook out for 1 min. Pour your chicken stock and whole milk into the pan and bring to a boil. Add your macaroni, salt, pepper `and oregano and turn down to a simmer. Cook for 10 mins until the macaroni is tender, stirring occasionally to prevent sticking.
Turn off the heat. Add your chicken and peas, then your cheddar, mozzarella and butter and stir to combine.
Roll out your pastry and trim into a large rectangle that will fit your baking dish and press a piece of macaroni around the edges to create a pattern.
Pour your mixture into a baking dish and top with your sliced mozzarella and the sheet of pastry. Brush with egg wash and poke a steam hole in the middle with a fork.
Bake for 25-30 mins until the pastry is risen and golden. Serve immediately!