Preheat the oven to 160°C.
Mix the crushed Tennis biscuits with melted butter and press into a pie dish to form the crust. Chill for 10 minutes.
In a bowl, whisk together the condensed milk, egg yolks, lemon juice, and zest until smooth and thickened.
Pour the filling over the biscuit base and spread evenly.
Beat the egg whites until soft peaks form. Gradually add sugar and cream of tartar, beating until stiff, glossy peaks form.
Spoon or pipe the meringue over the filling, sealing it to the edges.
Bake for 15–20 minutes until the meringue is lightly golden.
Cool completely, then refrigerate for at least 1 hour before serving.