Combine water, lemon juice, cornstarch and ¾ c sugar in pot over medium heat. Stir well to dissolve cornstarch, then add rhubarb.
Simmer for a couple of minutes, taste, and add more sugar or lemon if necessary.
Pour into baking dish or casserole.
Preheat oven to 400.
Combine flour, baking soda and powder, salt, and 2 T sugar in a bowl.
Whisk the melted butter into the buttermilk, then add it to the dry ingredients.
Mix minimally but well, until you end up with a sticky mixture.
Drop spoonfuls of dough on top of the rhubarb. (Because the dough is so sticky, the easiest way to do this is to take a spoonful and use another spoon to scrape it out.) You’ll end up with a lumpy biscuit that does not quite cover the fruit.
Bake for 20-25 minutes, until the topping is golden brown. Serve warm, topped with whipped cream, crème fraîche, or vanilla ice cream.