Preheat your oven to 375°F (190°C). Lightly grease a 9x9 inch baking dish or similar capacity oven-safe dish.

Prepare the filling: In a large bowl, gently combine the chopped rhubarb, quartered strawberries, granulated sugar, 3 tablespoons of flour, and vanilla extract. Mix until the fruit is evenly coated. Pour the mixture into the prepared baking dish.

Prepare the crumble topping: In a separate medium bowl, whisk together the rolled oats, 1 cup of flour, brown sugar, and salt.

Cut in the cold butter: Using a pastry blender, two knives, or your fingertips, cut the cold butter cubes into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces remaining.

Sprinkle the topping evenly over the fruit filling in the baking dish.

Bake for 40 to 45 minutes, or until the topping is golden brown and the fruit filling is bubbling thickly around the edges.

Let the crumble cool on a wire rack for at least 15 minutes before serving. Serve warm, optionally topped with vanilla ice cream or whipped cream.