To make the sauce
Preheat oven to 180C. In a large saucepan or in an ovenproof skillet, melt butter over medium heat. Add flour and cook, stirring constantly, for 1 minute. Add about 1 cup of the milk and mix to dissolve the paste into the milk. Then add the remaining milk and mix until it is lump free. Use whisk if required. Mix in the salt. Cook, stirring/whisking regularly, for 5 - 8 minutes until mixture is thickened to a cream consistency. When the Sauce coats the back of a wooden spoon, you should be able to draw a path with your finger. Remove the sauce from stove, add cheese and stir. The cheese doesn't need to melt. Adjust salt to taste.
Pour the sauce into a pot with the pasta. Stir quickly, then pour back into the skillet or a baking dish. Sprinkle with breadcrumb topping. Bake for 25 minutes or until the top is light golden. Serve immediately and if you want sprinkle with some parsley