Pre-heat the oven to 200°C / 400°F / Gas Mark 6.
To make mashed potatoes, cook the potato pieces in a pan with boiling water. Simmer for 10-15 minutes depending on the size of your pieces, until the potato is soft. Drain and mash with half fat crème fraiche, milk or cream will do. Add a twist of milled black pepper.
Meanwhile to make sauce, melt butter in pan and add flour. Stir with a wooden spoon and it will form a paste.
Gradually add the milk, this is easier using a balloon whisk but a wooden spoon will do, and keep stirring to ensure it is smooth.
Bring to boil to thicken and thoroughly cook the flour then reduce heat. Add the chicken and vegetables. Stir to combine. If the mix is a little thick, then add a little more milk and stir to blend in. Spread the mix in an ovenproof dish.
Top with the mashed potato and spread evenly. Use a fork to decorate.
Bake for 20 minutes until the potato topping is golden and the chicken mix is thoroughly heated through.