Boil your new potatoes in salted water for 12-15 minutes until cooked. Heat your oven to 180F/200C.
Meanwhile, heat the olive oil and the knob of butter in a pan and fry your leeks for 8-10 minutes until soft. Add the smoked sausage and fry for 5 minutes.
Whilst your leeks are cooking, make your cheese sauce by melting the butter in a pan, then add the flour to make a roux and then add the milk little by little, stirring all the time until you have a glossy, thick sauce that coats your spoon. Remove from the heat and add your cheese and seasoning and stir until melted in to the sauce.
Pour the sauce over the leeks and sausage and mix to combine. Pour into an oven dish.
Drain your cooked potatoes and then take a fork and gently crush each one so that the skin breaks a little. Pour over the top of the leek mixture and spread out to cover evenly. Drizzle a little olive oil over the top of the potatoes just to aid with the crisping up of the edges.
Bake in the oven for 15-20 minutes until the edges of the potatoes are crisp and golden. Serve.