Oxalic acid is where rhubarb derives its flavor– this is concentrated to toxic levels in the root and the leaf so discard those. If you have any problem with kidney stones rhubarb is not your friend– it binds calcium. So making a rhubarb custard pie actually makes sense. Some folk like the pie sweeter than the following recipe but the usual store rhubarb you buy is much lower in acid than the old-fashioned Victoria and such. All that said, here’s the recipe:
4 cups chunked rhubarb (3/4 inch pieces more or less)
1 1/4 cups sugar mixed with 2 TB flour and a dash salt
3 beaten eggs
1/4 tsp fresh ground nutmeg
1 Tb lemon
Toss the above together and bake in a regular pie crust, (with a top– lattice is pretty but I never have the time) at 400 Farenheit for about 50 minutes until tender.