Preheat your oven to 375°F (190°C). Lightly grease a 9x13 inch baking dish.

In a large bowl, combine the chopped rhubarb, cherries, granulated sugar, 1/4 cup flour, and vanilla extract. Toss gently until the fruit is evenly coated. Pour the fruit mixture into the prepared baking dish and spread it into an even layer.

In a separate medium bowl, prepare the topping. Whisk together the rolled oats, 1 cup flour, brown sugar, granulated sugar, cinnamon, and salt.

Add the cold, cubed butter to the oat mixture. Using a pastry blender, a fork, or your fingertips, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.

Evenly sprinkle the streusel topping over the fruit mixture in the baking dish.

Bake for 45-55 minutes, or until the topping is golden brown and the fruit filling is bubbly and tender. If the topping starts to brown too quickly, you can loosely tent it with foil.

Remove from the oven and let cool for at least 15-20 minutes before serving. This allows the filling to set slightly. Serve warm, perhaps with a scoop of vanilla ice cream, if desired.