Set oven to 350F

Rinse and trim the ends off the rhubarb stalks. Slice them in 1 inch pieces and put in a heavy pot. Toss with the sugar, lemon juice, and water. Stirring constantly, bring the mix to a boil and cook until the rhubarb has broken down and the mixture is thick. Add a little more water if needed as it cooks if it gets too thick. This will take about 20 minutes or so. You can puree it if you want a smooth texture, I used an immersion blender and pulsed a few times to get any larger lumps, but some texture is fine. Stir in the vanilla and set aside to cool.

To make the crust, put the dry ingredients in a food processor and pulse to combine. Add in the butter and pulse until the mix has a texture of coarse crumbs.

Reserve 1 1/2 cups of the mixture and press the rest into a buttered 9x13 baking pan. Pat down the crust and level it out.

Spread the filling evenly over the crust, and then crumble the remaining dough over the filling.

Bake for 30 minutes until golden.

Cool before cutting.