For the Pastry

• 170g plain flour
• 85g cool butter, cubed pinch of salt
• 3 tablespoons cold water
• 1 beaten egg or milk to wash the pastry

For the Filling

• 1 tablespoon olive oil
• 2 large red onions, sliced thinly into half moons
• 1 tablespoon caster sugar
• juice of ½ lemon
• 1 medium courgette, peeled into ribbons
• 250g smoked salmon
• 300ml double cream
• 1 bay leaf
• ¼ teaspoon nutmeg, freshly grated
• 1 teaspoon wholegrain mustard
• 3 egg yolks
• 1 whole egg, beaten
• 100g mascarpone cheese (optional)

Start with the pastry, and make sure everything is set up and ready to use. Pastry ingredients need to be cold to get the best finish so work quickly. However, I find it is best to take the butter out of the fridge 10 minutes before you need it or you have to work it for longer and warm it up by accident. I put the water into the fridge at the same time.

Sift the flour into a large bowl and add the cubed butter and the salt. Gently rub the butter into the flour with your fingertips until it resembles fine breadcrumbs. Lift the crumbs up in the bowl and let them fall again to keep the air in the mix.

Add the water, a tablespoon at a time, scattering it over the mix evenly, and bring it all together with your hands. The dough should be firm, but not sticky. You may need a drop or two more water if it looks dry. Once the dough comes together in a lump, leaving the bowl clean around it, wrap it in cling film and chill for 30 minutes to relax the gluten and make it easier to roll out.

Preheat the oven to 190°C. Roll the dough out on a floured surface: you want a circle big enough to fit a 23cm tart tin and about 4–5mm thick. Butter and grease your tin.

Wrap the pastry round the rolling pin and lift it into the tart tin. Using your fingers, lift and press the pastry snugly into the tin. Don’t trim it at this stage: pastry shrinks when cooked and, if you trim it, you risk not having enough depth for the filling. Prick the base a few times with a fork.

Line the pastry with greaseproof paper and fill it with dried rice or baking beans. Bake it ‘blind’ like this for about 15 minutes. The baking beans stop the pastry raising up. After 15 minutes, remove the greaseproof paper and baking beans. Brush the egg or milk over the pastry and return it to the oven for about 5–7 minutes until the pastry is lightly browned. Cool on a wire rack while you make the filling.

Reduce the oven temperature to 175°C.

Heat the olive oil in a frying pan over a low heat, then add the onions and soften until they are translucent. This should take about 10 minutes. Add the sugar and lemon juice to help the onions to caramelise and cook them out for another 20–25 minutes until they are a deep golden colour. Scatter them over the bottom of the tart case.

Turn your courgettes into ribbons using a potato peeler: this makes them look lovely and allows them to cook more easily too.

Arrange the ribbons of courgette and the slices of smoked salmon over the onions in the tart case so they are evenly distributed. Don’t chop the salmon. You want bite to the tart.

Gently warm the cream in a small pan with the bay leaf, nutmeg and mustard over a low heat. Add the egg yolks one at a time, whisking them in well, and then the beaten whole egg. Whisk the mixture thoroughly and season, bearing in mind the saltiness of the fish when doing so.

Pour the custard into the tart tin. Finish with spoonfuls of the mascarpone (if using). Check you have reduced the oven temperature to 175°C and bake the tart for 30 minutes. The custard needs to set, but shouldn’t scramble, so don’t be tempted to overcook it. There should be the faintest wobble to the centre of the tart.

Remove the tart from the oven. Allow to cool for a minute or two and then set the tin on an upturned bowl and ease the tart out. Cool for 5 minutes and then serve warm with dressed salad leaves or potato salad. It can also be served cold at a picnic.

You could use shop bought pastry for the tart, but Ella’s pastry is foolproof.