Combine flour, granulated sugar, and oats. Cut in butter until a coarse, crumble topping forms.
Add the rhubarb, brown sugar, ¼ cup of water, vanilla and cinnamon to a saucepan and bring to a simmer.
Make a slurry with the remaining 1/4 cup of water and cornstarch. Pour the slurry into the rhubarb mixture.
Stir continuously for 2 to 3 minutes until thickened. Remove from heat.
Transfer the mixture to a prepared baking dish.
Sprinkle the crumble topping over the rhubarb mixture.
Baked on 400°F for 30 to 35 minutes until golden brown.
Serve warm with vanilla ice cream.
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