Preheat oven to 350 F.

In a large mixing bowl, combine the flour, oats, brown sugar, and cinnamon.

Mix in the butter with your fingers until crumbly.

Press half of the crumbs in the bottom of a greased 9 x 9 pan.

Set remaining crumbs aside.

In a saucepot over medium-high heat, stir together the sugar and cornstarch.

Add the water and vanilla. Bring to a boil while constantly stirring, until thick and smooth.

Remove from the heat and stir in the rhubarb and apple, tossing to coat. Pour fruit over crust and top with remaining crumbs.

Bake for until bubbly and golden brown, about 35 to 40 minutes. Serve warm with ice cream.