These pies are very popular in Scotland. Basically, you take a hard pastry shell (as used in Scotch pies), fill it with macaroni cheese and bake it. Pure, unadulterated comfort food.
You'll need some kind of rings or ramekins to form your shells. They should be 9-10mm (3½-4 inches) diameter, and 3cm (1¼ inch) tall. Butter the inside of your formers. Sprinkle flour on your worktop and roll some dough as thinly as you can. Seriously, it'll be hard work, but the thinner you can get it, the better. Now sprinkle a generous amount of flour on the pastry disc and lay it into a former, floured side down. Use a blob of spare dough to press the pastry right down into the bottom of the former. Try not to get any folds or creases in the pastry. Roughly cut off the excess dough around the top - don't cut it right up to the rim at this stage. Place the shells in the fridge to dry out for between 3 hours and three days.
Bring a pan of salted water to a rolling boil. When it's boiling, add the macaroni and cook it for a minute less than the manufacturer's recommendation. Stir it once of twice to stop it from sticking to itself. Drain and set aside until needed.
Start making the sauce as soon as you've got the pasata going. Melt the butter in a medium frying pan. Stir in the flour and cook for a minute or two. Gradually add the milk (if you've done the infusion, remove the onion). Stir it until it starts to thicken, then stir in the cream and mustard. Gradually add most of the grated cheese but leave a bit to sprinkle on the pies before baking. Now add the pasta. When it's all combined, add a bit more milk if it's too thick. Give it a taste and add salt if it needs it, although most cheese is quite salty so you may not need to add extra.
Let the sauce cool down before filling the pies.
MAcaroni pies are brilliant hot or cold.