Press your potatoes through a ricer into a large bowl. Add your eggs, cream, parmesan and salt and stir with a spoon until the ingredients are totally combined.
Put your ingredients in a piping bag fitted with a large star nozzle. Place a large greased cake tin upside down on a baking tray, then carefully pipe around the sides and top until totally covered. Bake at 200°C/400°F for 30 mins until you have a crisp potato shell. Gently remove from the cake tin and place on a rack to cool.
Combine your chicken with vegetable oil, salt, pepper and thyme in a bowl until salt the pieces are coated. Heat a frying pan on a medium-high heat and tip in your chicken breast chunks. Fry for about 5 mins, until golden on the outside. Set aside.
Heat your butter in a large saute pan over a medium heat, then add your onion and cook for 3 mins until starting to soften. Add your carrot and celery and cook for a further 5 mins until all softened. Throw in your garlic and cook out for 1 min before adding your flour to the pan and stirring to coat your vegetables, cooking out for about a minute longer.
Pour in your chicken stock and stir to create a smooth sauce. Bring to a boil, then simmer for 2 mins until your sauce thickens.
Pour in your cream, then add your chicken back into the pan along with your peas and stir to combine. Simmer for 5 mins until your sauce thickens slightly and your chicken is fully cooked. Add the cheddar, parsley, oregano, salt and pepper and stir to combine.
Place your potato case back on a lined baking tray and sprinkle the base with mozzarella. Pour your chicken pie mix into the potato case, then top with half of your pastry strips. Brush with egg wash, then bake for 25 mins until the pastry is golden and cooked through.