Oatmeal-Rhubarb Bars

1 cup flour
3/4 cup oatmeal -- uncooked
1 cup brown sugar -- packed
1/2 cup butter, unsalted -- melted

1 cup sugar
2 teaspoons cornstarch
1 cup water
1 teaspoon vanilla extract
4 cups rhubarb, 1/2" pieces

Mix flour, oatmeal, brown sugar and butter until crumbly. Press 1/2 into greased 9" pan. Add rhubarb, cut into 1/2" pieces. Combine sugar, cornstarch, water and vanilla; cook till thick and clear. Pour sauce over rhubarb. Top with other half crumb mixture. Bake at 350F for 45 minutes.