- Crust:
- 1 cup all-purpose flour
- 1 cup brown sugar
- 3/4 cup old-fashioned oats
- 1/2 cup butter, melted
- 1 teaspoon ground cinnamon
- 4 cups chopped rhubarb
- Sauce:
- 1 cup water
- 1 cup white sugar
- 2 tablespoons cornstarch
- 1 teaspoon vanilla extract
- Preheat oven to 350 degrees F (175 degrees C).
- Mix flour, brown sugar, oats, butter, and cinnamon together in a bowl until crumbly; press into the bottom of a 9-inch square baking dish. Spread rhubarb in an even layer over the crust.
- Stir water, sugar, cornstarch, and vanilla extract together in a saucepan over medium low heat; cook, stirring occasionally, until thick and clear, 5 to 10 minutes. Drizzle sauce over rhubarb.
- Bake in preheated oven until topping is set, about 1 hour.