You’ll need a 22cm tart tin with a removable base.

If the leeks are dirty, chop them as per the recipe, put them in a large bowl of cold water and swish with your hands: the mud should fall to the bottom of the bowl. Scoop out the clean leeks with your hands, dry them on a clean tea towel and use from there.

Heat the oven to 200C (180C fan)/390F/gas 6 and take the pastry out of the fridge.

Put the butter and olive oil in a wide frying pan on a medium heat. When it bubbles, add the shallots and rosemary leaves, and cook, stirring occasionally, for five minutes.

Add the leeks and cook for a further five minutes, gently prodding the leek rings so they unravel.

Add the garlic and fine salt, and cook for another four minutes.

Stir in the cream and mustard, let everything bubble for a couple of minutes, then take off the heat.

While the leek mixture cools slightly, prepare the pie tin. Cut a square of pastry 2cm larger than the top of the tin and gently lay it inside. Using a sharp knife, trim and discard any overhang. Push the pastry into the base and sides of the tin, and press up the edges.

Pour in the leek mixture and spread it out evenly. Layer the potato slices on top, overlapping them slightly, then brush all over with olive oil and sprinkle with sea salt. Pop the remaining rosemary sprig in the centre, then bake for 40-50 minutes, until the top is browning in places.

Remove from the oven and leave to cool for five to 10 minutes.

To unmould the tart, put the tin on an inverted bowl with a circumference slightly smaller than the hole at the bottom of the tin. Carefully slide the ring down and off the tart, then slice and serve with a sprightly herb and leaf lemon-and-oil-dressed salad.