1. To make the curd: zest and juice the lemons. Set the zest to one side. Place the curd ingredients into a heatproof bowl (if you’re making the curd before the meringue, save the whites for the meringue). Set the bowl over a pan of simmering water (take are not to let the bottom touch the water). Place on a gentle heat and whisk till the butter has melted and the curd is thick enough to coat the whisk. This could take 20-30 mins. Stir the zest in. If the eggs start to scramble, take the bowl off the heat straight away- it has got too hot. Keep whisking and it should cool down and come together.
2. Pour the thickened curd into a sterilised jar or if making straight away, a tub is fine. Leave to cool fully before filling your meringue. This will make a little more than you need for this recipe. It is delicious spread on hot toast or saved to be sandwiched between Victoria sponges.
3. To make the meringue: Preheat your oven to 220°C/Fan 200°C/Gas 7. Line a swiss roll tin (approx. 35x30cm) with baking parchment allowing 2cm to overhang on all edges.
4. Make sure a large mixing bowl is clean and grease free as this will stop the egg whites from whisking. Separate the eggs, placing the whites into the mixing bowl and the yolks into heatproof bowl to make the curd.
5. Using an electric mixer or electric hand whisk, whisk the egg whites till they form soft peaks when you lift the whisk out of the bowl.
6. Still whisking on a medium speed, add a tablespoon of sugar at a time making sure it is incorporated before you add another. When all the sugar is used, whisk for 5 mins.
7. Turn your mixer to a low speed then add the lemon zest, vinegar and corn flour. Mix for 1 min.
8. Scrape the meringue mix on to the lined baking tin and gently spread out to cover the tin.
9. Slide the tin into the oven and immediately lower the temperature to 200°C/Fan 180°C/Gas 6
10. Bake for 30 mins till crisp on top and lightly golden.
11. Remove from the oven and leave to cool. It will collapse a little. Cover with a tea towel. You can make this a day in advance, keeping it covered .
12. Just before you are ready to serve, whip the double cream up with the elderflower cordial till it stands in soft peaks when you lift the whisk. Take care not to over whisk.
13. Flip the meringue carefully on to a clean tea towel and remove the baking paper. Carefully place the meringue onto a large serving dish or tray.
14. Ripple half of the reserved curd through the whipped cream.
15. Spread two thirds of the cream and curd mix over the meringue, all the way to the edges. Scatter over half of the berries.
16. With the long side of the meringue closest to you, roll the meringue away from you, using the tea towel to help roll it. The meringue will crack slightly, but don’t worry. It is held together by the cream filling.
17. Dredge over some icing sugar. Dollop the remaining cream on top and roughly spread over. Dot over some lemon and elderflower curd then scatter over the remaining berries. Decorate with the elderflower heads of edible flowers if using. Serve straight away. Any leftovers will keep covered in the fridge for 2 days.