Step 1

Preheat the oven to 200°C/fan 180°C/gas Roll out the pastry on a dusted surface to the thickness of a pound coin, then use to line a 20cm fluted tart tin. Chill for 10 minutes.

Step 2

Line the pastry case with baking paper, then fill with baking beans or rice and blind bake for 15 minutes.

Step 3

Remove the baking paper and beans/rice, then bake the pastry for a further 10 minutes or until golden. Remove the tart tin from the oven and set aside. Turn the oven down to 180°C/fan160°C/gas 4.

Step 4

In a frying pan, heat the butter and oil, then add the onion and leeks. Cook over a low heat for 10 minutes or until soft and translucent, but not coloured. Set aside to cool.

Step 5

Meanwhile, in a jug, mix the eggs with the cream, mustard and grated gruyère. Season well, then add the cooled onion and leeks and stir well. Pour the mix into the tart case, top with the sliced gruyère, then bake for 25-30 minutes until golden. Boom! An incredible tart. Serve with a leafy side salad with a mustard dressing.