1 Line a 24–25cm (9½–10in) deep round loose bottom tart tin.

2 To make the pastry, put the flour, butter and Parmesan in a food processor. Whizz until it looks like breadcrumbs. Add the egg and 1 tablespoon of water and whizz until the mixture forms a ball. Roll out on a clean, lightly floured work surface and line the base and sides of the flan tin with the pastry. Set aside to chill in the fridge for 30–35 minutes.

3 Preheat the oven to 220°C/425°F/Gas7. Prick the base of the flan pastry and line with baking paper and then baking beans. Blind bake the pastry for 10 minutes. Remove the beans and bake for a further 8–10 minutes until lightly golden, then place the pastry case on a baking sheet.

4To make the filling, melt the butter and olive oil in a frying pan. Add the onions and fryover a medium– high heat for 5 minutes. Cover with a lid and gently simmer for 20 minutes until soft and golden brown. Spread onto the base of the tart.

5 Beat the eggs in a bowl. Add the cream and milk and season. Add the Gruyère,then pour into the case.Sprinkle with the Parmesan.

6 Reduce the oven to 180°C/350°F/Gas4. Bake for about 30–35 minutes until lightly golden on top and set in the middle.