Dice the chicken into bite-sized pieces and season with ¼ tsp salt and black pepper. Add a drizzle of oil to a large pan over medium-high heat and add the chicken. Sear both sides until golden (don't worry too much about cooking it right through at this point) then remove from the pan.

Lower heat slightly to medium, then add in the bacon. Fry until crisp with the fat rendered, then remove all but 1 tbsp of the fat. Add the leek and fry until soft and starting to turn golden.

Melt in 1 tbsp butter, then stir in 2 tbsp flour. Gradually pour in the stock, stirring as you go to prevent lumps from forming. Stir in the cream, crème fraîche, mustard & thyme then simmer for 8-10mins until it's nice and thick. Check for seasoning and adjust if needed (go easy with the salt) then pour into a baking dish. Leave for at least 5mins before topping with mash (helps form a 'skin' to form a barrier).

Meanwhile, add the potatoes to cold water with 2 tsp salt and boil for around 15mins or until fork tender. Drain and leave for a few mins to allow some steam to escape, then mash with butter and cream. Stir in cheddar and ½ tsp salt & ¼ tsp black pepper (or to taste).

Top the filling with the mash, then rough the top with a fork. Place the dish on a tray then bake in the oven at 200C/390F for 30mins, or until golden and crisp on top with the gravy bubbling around the sides. Leave for 5mins or so before serving to help it hold its shape.