Preheat the oven to 200°C/Fan
180°C/Gas 6 and butter a deep-sided,
medium ovenproof dish. On a medium
heat, pan-fry the onion in the olive oil
for 5–6 minutes until softened and
translucent. Turn the temperature
down a little and add the balsamic
vinegar and brown sugar. Keep
stirring for 3–4 minutes until the
onions are coated and sticky, then
remove from the heat and set aside.
In a large saucepan, melt the butter
and add the flour, whisking constantly
to make a roux. Then add the milk in
stages, continuing to whisk to prevent
lumps from forming. Keep whisking
until the sauce thickens, then add the
double cream, 1 teaspoon of salt and
150g of the grated cheese, and stir
until the cheese is melted. Add the
glazed onions to the cheese sauce and
combine well.
Meanwhile, bring a large pan of
salted water to the boil, add the
macaroni and cook until al dente –
usually a minute or two less than the
cooking time given in the packet
instructions. Drain the pasta, then tip
into the pan with the cheese and onion
sauce, and mix well.
Pour the cheese and onion pasta
into the prepared ovenproof dish,
sprinkle over the rest of the cheese
and bake in the oven for 30–35
minutes until bubbling and browned
on top. Serve with plenty of black
pepper ground on top.