Preheat the oven to 200C.
Heat some cooking oil in a large frying pan, cook the chicken and bacon over a high heat until the chicken has no more pink bits and the bacon starts to turn crispy.
Add in the leeks and cook for a few minutes, until they start to soften.
Add the plain flour and stir well. Cook for one more minute.
Pour in the chicken stock and stir well. Then add the crème fraiche, mustard, and frozen peas. Stir well and bring it to a simmer.
Taste for seasoning, then spoon it into an oven proof dish.
Slice the potatoes to ½ cm thick, then layer them over the creamy sauce. Scatter over the diced butter.
Bake for 45 minutes, until the potatoes are golden and crispy. Leave to stand for a few minutes before serving and enjoy.