Peel the potatoes, cut in half and boil until just cooked (you don’t want to overcook them as you want them to hold their shape so that you can slice them). This will take about 15-20 mins depending on their size.
Cook's tip: for babies, purée, mash or finely chop the bake.
Trim the leeks and wash, then cut into thin slices. Heat the butter and oil in a large saucepan, add the leeks and fry gently until really soft, this will take a good 10 mins.
Add the flour, stir in and cook for a few more minutes.
Add the milk and keep cooking until you have a smooth sauce.
Remove the skin from the chicken and cut into chunks. Add to the sauce with the peas. Then pour this into an ovenproof dish.
Slice the potatoes as thinly as possible and arrange a over the top of the sauce. Dot the top with the remaining butter.
Bake in the oven for 35-40 mins until slightly golden on top and all bubbling.