Make the pastry: Add the flour and grated butter to a large bowl and rub together until the texture resembles breadcrumbs. Add the sugar, salt, and water and rub together a little more, but do not overmix! Tip out onto a clean surface and bring together into a ball. Wrap in cling film and let rest in the refrigerator for 10 minutes.
3. Step
Roll out the chilled pastry to ½ cm thick, then gently transfer to a 9-inch tart pan, trimming the excess pastry around the edges (you may roll a little of the excess dough into a ball and use this to push the pastry into the edges of the tin). Prick the bottom of the crust with a fork, line with parchment and baking weights, and bake for 15-20 minutes. Remove from the oven and let cool.
4. Step
Make the lemon curd: Combine the lemon juice, cornflour, sugar, turmeric, oat milk, and lemon zest in a large saucepan and whisk over medium-low heat until it thickens. Remove the pot from the heat.
5. Step
Make the meringue: Squeeze the lemon juice into the metal bowl of a stand mixer, then use the lemon half to coat the inside of the bowl evenly. Add the aquafaba and whip with the whisk attachment on high speed for 5 minutes, until it quadruples in volume.
6. Step
Add the sugar and vanilla and mix again on high speed for 5 minutes, until glossy and the sugar has dissolved.
7. Step
When the pastry is cool, fill three-quarters of the way with the lemon curd, then let cool for 20 minutes. Dollop the meringue over the curd and spread a little with a spatula to make grooves, which will caramelize into lovely browned marks.
8. Step
Broil for 3–55 minutes (keep an eye on it though to ensure it doesn’t burn!),until the top of the meringue is browned.
