Boil pasta in plenty of salted water according to manufacturer’s instructions. Drain and toss with a little oil. Set to one side.
To make the sauce
Melt the butter in a saucepan and then stir in the flour to make a roux. Remove from the heat and gradually stir in the milk.Return to the heat and stir continuously until thickened. Whisk in the mustard and season to taste. Set to one side.
For the meat and leek mixture
Heat the olive oil and lightly fry the sliced leeks until soft.
Remove the skins from the sausages and break up each sausage into small chunks. Add to the leeks and fry until cooked and golden brown.
Deglaze the pan with a splash of white wine or water. Add the sausage mixture to the pasta and mix well.
Place into a baking dish.Pour the white sauce over the pasta and sausage mixture, mixing well.Mix together the breadcrumbs, grated cheese and paprika and sprinkle over the top of the pasta.