Combine flour, oatmeal, cinnamon, and brown sugar into a medium bowl and mix with a whisk.

Pour the melted butter into the oatmeal mixture and stir until combined. 

Pour half of the mixture into a greased 8x8" glass pan.

Combine the sugar, water, cornstarch, and vanilla in a small pot, and cook slightly below medium until very thick. Add the rhubarb to the pot and mix thoroughly.  

Spread the rhubarb mix into the pan and sprinkle the remaining oatmeal mixture on top. 

Bake at 350 for 1 hour. Serve warm or cold.