Preheat the oven to 350˚
Make the Crust/Topping:
mix the dry ingredients together
cut the butter in chunks and drop it in the dry mix, cut in with a pastry blender or two knives, or use your hands mix it in
keep mixing until it looks like coarse crumbs
save off 1 cup of crumbs
press the rest in the bottom of a 9X13 baking dish
Make the Rhubarb Filling
put the rhubarb, water (if using) and sugar in a sauce pan
bring it to a boil over medium high heat, it will get very juicy as it cooks
once it comes to a boil reduce the heat to keep it from scorching, just keep it bubbling lightly until the rhubarb is just starting to get soft
remove 2/3 cup of the juice and whisk in the flour into it
add it back in the pan and stir until thickened, cook and stir for 1-2 minutes and remove from the heat (you just made rhubarb gravy!)
spread the rhubarb over the base layer in the 9X13 sprinkle with reserved crumbs
bake for 30-35 minutes until top is lightly brown and the rhubarb is bubbly
let it cool before you cut it for best results