Step 1: Preheat your oven to 375°F (190°C). Lightly grease a 9x9 inch baking dish or a 2-quart casserole dish.
Step 2: In a large bowl, combine the chopped rhubarb, mixed berries, granulated sugar, cornstarch, vanilla extract, and optional cinnamon. Toss gently until the fruit is well coated. Pour the fruit mixture into the prepared baking dish.
Step 3: In a separate medium bowl, combine the flour, rolled oats, brown sugar, and salt for the topping.
Step 4: Add the cold, cubed butter to the dry ingredients for the topping. Using your fingertips, a pastry blender, or a fork, cut the butter into the mixture until coarse crumbs form. Avoid overmixing; some larger pieces of butter are desirable.
Step 5: Evenly sprinkle the crumble topping over the fruit mixture in the baking dish.
Step 6: Bake for 40-45 minutes, or until the topping is golden brown and the fruit filling is bubbly around the edges. If the topping browns too quickly, you can loosely tent it with foil.
Step 7: Remove from the oven and let it cool for at least 15-20 minutes before serving. This allows the filling to set. Enjoy warm with a scoop of vanilla ice cream or a dollop of whipped cream!