Preheat the oven to 350˚
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's looks like coarse crumbs
save off 1 cup of crumbs
press the rest in the bottom of a 9X13 baking dish
Make the Filling: put the rhubarb, water (if using) and sugar in a saucepan
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
add the sliced strawberries and cook for one more minute or so
remove 2/3 cup of the juice and whisk in the flour for the filling 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 strawberry rhubarb mixture over the base layer in the 9X13 sprinkle with reserved crumbs
bake for 30-35 minutes until top is lightly brown and the strawberry rhubarb layer is bubbly
let it cool before you cut it for best results