- 1 1/2 cups all-purpose flour
- 1 1/4 cups rolled oats
- 1 cup brown sugar
- 1 teaspoon ground cinnamon
- 1/2 cup melted butter
- 2 cups chopped rhubarb
- 2 cups chopped apples
- 2 tablespoons cornstarch
- 2 tablespoons water
- 1 cup white sugar
- 1 cup water
- 1 teaspoon vanilla extract
- Preheat oven to 325 degrees F (165 degrees C).
- Stir flour, oats, brown sugar, and cinnamon together in a bowl. Pour melted butter into oat mixture and stir until crumbly. Pat half the oat mixture into the bottom of a 9×13-inch baking dish.
- Stir rhubarb and apples together in a bowl; pour on top of oat mixture in baking dish.
- Stir cornstarch and 2 tablespoons water together in a bowl until cornstarch is dissolved and mixture is smooth.
- Heat sugar, 1 cup water, and vanilla extract in a saucepan over medium-high heat; bring to a boil, stirring constantly. Pour cornstarch mixture into sugar mixture and continue to boil, stirring constantly, until mixture thickens, 1 to 2 minutes. Drizzle sugar mixture over rhubarb and apples; sprinkle remaining oat mixture over the top.
- Bake in the preheated oven until hot and bubbly, 35 minutes.