Add chopped potatoes ( parsnips or kumera could be added) to a large pan of water. Boil until soft ( check with fork). Drain and mash with a little seasoning
Melt butter in a large pot and cook leeks and mushrooms on a gentle heat about 10 mins. Add herbs, stock and chicken bring to a simmer until chicken is cooked.
Mix cornflour to a smooth paste with a little of milk. Add to chicken in pan with remaining milk and stir until slightly thickened. Season generously. Pour into buttered oven proof dish.
Place mashed potato on top of chicken, leeks and mushrooms. ( Filo pastry could also be used here ).
Can be served immediately, or heated up later in the microwave or oven.