Preheat oven to 350˚F.

In a large mixing bowl, stir together the flour, rolled oats, brown sugar, melted butter, and cinnamon until crumbly. Press half of the crumb mixture into a buttered 9” round cake pan. Cover with diced rhubarb.

In a small saucepan, combine sugar, cornstarch, water and vanilla. Heat over a medium flame, stirring frequently, for 3-5 minutes, until sugar is dissolved. Pour the liquid into the cake pan, coating the rhubarb. Top with remaining crumb mixture.

Put in the center of the oven and bake for one hour.

Let cool on a wire rack, then serve.