Line the base and sides of a deep 20cm springform cake tin with baking parchment. Put the biscuits, butter and coconut oil in a food processor and blitz until the mix resembles fine breadcrumbs. Tip into the tin and press into the base and up the sides. Chill for 2 hrs.
To make the filling, heat the lemon zest and juice, 100ml water and the sugar in a pan set over a low heat, stirring until the sugar dissolves. Stir in the turmeric and a pinch of salt. When the sugar has dissolved, remove the pan from the heat and spoon a little of the mix into a small bowl. Combine with the cornflour to make a paste, then tip into the pan, whisking continuously. Return the pan to the heat and bring to the boil, whisking. Cook for 1 min.
Remove the pan from the heat and whisk in the cream and butter. Pour the mixture over the base, and return to the fridge. Chill until set, about 30 mins.
Heat the oven to 140C/120C fan/ gas 1. Put all the ingredients for the meringue in a large, clean bowl. Beat with an electric whisk for 10-12 mins, or until glossy (it will be quite stretchy). It will take longer to whip than egg whites, so don’t be put off if it’s still liquid after 5 mins.
Pipe or spoon the meringue onto the tart. Bake on the middle shelf of the oven for 1 hr 30 mins. Leave to cool completely in the tin, then serve.