Preheat oven to 180°C (Gas Mark 4, 350°F). Place the flour, salt and sugar into a mixing bowl and add the non-dairy butter. Using your fingertips, rub the butter into the flour until you have a mixture resembling fine breadcrumbs.
Drizzle over the water and briefly mix in to the flour mix. Bring the pastry together into a ball with your hands. Wrap in cling film and chill in the fridge for 10 minutes to firm up.
Place the unwrapped pastry on a floured surface and roll out to the thickness of a one pound coin. Drape the pastry over your tin and gently push into all the edges. Use your rolling pin to roll over the tin edge to remove excess pastry. Line the pastry base with foil and transfer to the oven for 15 minutes.
To make the filling, combine the sugar, cornflour, salt, soya milk and water. Heat gently, stirring continuously, until thick. Add the lemon and lime juices and zests and stir well to combine. Set aside to cool.
Lower the oven to 150°C (Gas Mark 2, 300°F). Pour the aquafaba into a large, clean bowl and add the cream of tartar. Using an electric hand whisk, beat the aquafaba until you have stiff peaks. Gradually add the sugar, a tablespoon at a time, whilst continuing to whisk, until it is all incorporated.
Pour the cooled filling into the pastry case and spread out evenly. Top with spoonfuls of the meringue, smoothing out with the spoon so there are no gaps remaining. Use the back of a spoon to make waves and peaks on the surface of the meringue.
Transfer the pie to the oven and bake, without opening the door, for 40 minutes. When cooked, serve warm, or leave in the oven to cool with the door ajar to allow the meringue to dry out.
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