For the filling:
12 ounces fresh rhubarb, diced
1-1/4 pounds apples, peeled, cored, and diced
2/3 cup granulated sugar
2 T flour
1 tsp vanilla
1/2 tsp cinnamon
For the topping:
2/3 cup rolled oats
6 T flour
1/3 cup light brown sugar, packed
5 T unsalted butter, cold, cut into small cubes
1. Preheat the oven to 375 degrees and spray a 6-cup baking dish with oil (you can also use other dish sizes if that's what you have).
2. Combine all of the filling ingredients in a bowl, and stir to combine. Transfer to the baking dish.
3. Stir together the oats, flour, and brown sugar (you can use the same bowl). Add the butter, and rub in with your hands until the mixture begins to hold together when compressed. Sprinkle over the fruit mixture.
4. Bake 45-55 minutes, until bubbling and tender. Cover loosely with foil if the topping gets brown to quickly. Cool 15 minutes before serving.