Put the potatoes into a large pot of salted water and bring to the boil.
Cook for 8-10 minutes until the potatoes are tender but not fully cooked through
Strain into a colander and allow to cool.
Meanwhile heat a large shallow saucepan with the butter (and a little bit of oil to stop the butter from burning). Add in the leeks, garlic and mushrooms at this stage together with a little seasoning and cook over a medium heat for 5 minutes. Sprinkle the flour over the cooked vegetables, then pour in the white wine and milk, stir to combine. Reduce the heat and add the turkey and ham, cook for 8 - 10 minutes. If you like you could add a little cream as well. Season lightly and add the wholegrain mustard. Simmer for 5-6 minutes and then transfer to a large casserole dish.
Neatly arrange the sliced potatoes on the top, Sprinkle with some salt, pepper, cheese and breadcrumbs and bake in a preheated oven (190C/375F/Gas Mark 5) for approximately 30 minutes.