Boil the peeled 2cm potato chunks in water for 15 minutes until tender.
Drain the potatoes and mash until smooth, then allow to cool for 15 minutes.
Brown the seasoned chicken thigh pieces in oil and transfer to a casserole dish.
Preheat the oven to 200°C.
Fry the bacon, onions, and celery for 5 minutes before adding mushrooms for 1 minute.
Add the vegetables, tomatoes, stock, herbs, and carrots to the casserole dish.
Bring to a simmer on the hob and bake covered in the oven for 30 minutes.
Rub butter into flour and salt, then incorporate 250g of mashed potatoes.
Mix in the milk to form a soft dough.
Roll the dough into a 24cm sausage and cut into twelve 2cm thick rounds.
Stir the cornflour Slurry and leeks into the hot chicken mixture.
Layer the potato rounds over the chicken and brush with milk.
Bake uncovered for 30 minutes until the topping is golden-brown and risen.