Bring 4 pints (2.3 L/10 cups) of water to the boil with 1 teaspoon of salt. Sprinkle in the macaroni and stir to make sure it doesn’t stick together; cook until just soft, 15 minutes approx. Meanwhile melt the butter and sweat the onions gently in it until soft, 8-10 minutes approx. Add in the flour and cook, stirring occasionally for 1-2 minutes. Blend in the milk gradually and bring back to the boil; keep stirring. Add ¼ teaspoon of mustard, the parsley, salt and freshly ground pepper to taste, and 2½ ozs (70 g/generous ½ cup) of the cheese. Stir in the cooked macaroni and turn into a pie dish. Sprinkle the top with the remaining cheese. Reheat in a hot oven, 200°C/400°F/regulo 6, or under the grill until the top is brown and bubbly.
This is good served with cold meat, particularly ham.