Preheat the oven to 180°C (350°F) (gas 4)
Heat the oil in a large frying pan/skillet. Add the onion and cook over a low heat for 4–5 minutes until softened but not browned.
Then add the beef and brown over a medium heat for about 3–4 minutes. You want it to just start turning colour.
Now stir through 5 tablespoons of the sauce and then leave to cool for a few minutes.
Spoon a layer of sauce over the bottom of the ovenproof dish.
Next, fill the cannelloni shells with the meat and lay them on top of the sauce in a single layer.
Pour the remaining sauce over the shells.
Next, to make the white sauce, melt the butter in a small pan, then tip in the flour and mix to a smooth paste.
Slowly add the milk, stirring continuously, until there is a smooth sauce. Bring to the boil. Reduce the heat to medium and let it simmer, while stirring, until the sauce starts to thicken slightly.
Season and add a little grated nutmeg.
Pour the sauce over the cannelloni and then sprinkle over the cheese.
Bake for 40 minutes until the top begins to brown and the sauce is bubbling.