Preheat the oven to gas 4 / 180C / 350°F
To make the filling, add all of the ingredients to a saucepan, checking the instructions for your gelatin / agar agar for how much you'll need to use and how to use it. I used pork gelatin sheets and needed 6 sheets to set the 720mls of liquid (3 cups) used in the filling. With agar agar you need to boil it to activate it. With gelatin you just simmer it or it won't set. Either way whisk it until it's thickened then it's ready
Keep the filling covered but off the heat while you make the meringue:
Pour the canned bean water into a stand mixer bowl and beat until cloud-like. Add the cream of tartar powder then the sugar in a tablespoon at a time
It should be thick enough that the peaks don't flop over when you pat the top with the back of the spoon
As soon as the meringue is done, pour the filling into the pie crust. While the filling is hot, spread the meringue mixture over it, carefully sealing to the very edge of the crust
Bake for 20 minutes or until the meringue has started to turn golden, then let the pie cool. If you use agar agar as the setting gel, it will set at room temperature. If you use gelatin you'll need to refrigerate it but let it come to room temperature to serve
