Peel and dice potatoes into even chunks. Boil in the vegetable stock then when done mash with 35g butter and set aside to cool.
Meanwhile gently heat the olive oil in a large frying pan. Fry the chicken until sealed on all sides. Put into a bowl.
Melt the remaining butter in the frying pan over a low heat and then sweat the finely sliced leeks for 3-5 minutes.
Put the chicken back in with the leeks with the chicken stock pot and creme fraiche stirring continuosly. Add 100ml of boiling water and stir. Remove from the heat after a couple of minutes and the sauce has thickened. Put into a casserole dish to cool.
When cooled spread the mashed potato over the chicken mixture and fluff up with a fork. Put grated cheese over and add black pepper. Put in oven 180, 170 fan and gas mark 5 for 30 to 35 minutes until bubbling and the topping has turned golden.
Ensure chicken is even sized so that it cooks properly.
Salt should not be needed because of the stock pots/cubes.
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