Step 1: Prepare the Fruit Filling

Preheat your oven to 350°F (175°C). In a large mixing bowl, combine the chopped rhubarb and strawberries. Sprinkle with the granulated sugar, cornstarch, lemon juice, and vanilla extract. Gently toss the fruit until everything is well combined. The cornstarch will help thicken the fruit juices as they bake.

Pour the fruit mixture into a 9×13-inch baking dish or a deep pie dish, spreading it out evenly.

Step 2: Make the Oat Crumble Topping

In another mixing bowl, combine the rolled oats, flour, light brown sugar, ground cinnamon, and salt. Pour the melted butter over the mixture and stir until the ingredients are fully incorporated and form a crumbly texture. The oat crumble should clump together but still be loose enough to sprinkle over the fruit.

Step 3: Assemble and Bake

Sprinkle the oat crumble topping evenly over the strawberry-rhubarb mixture, making sure to cover the fruit completely. Place the baking dish in the preheated oven and bake for 40-45 minutes, or until the topping is golden brown and the fruit filling is bubbling around the edges.

Step 4: Cool and Serve

Once the crisp is done baking, remove it from the oven and let it cool for 10-15 minutes to allow the filling to set slightly. Serve the crisp warm, topped with a generous scoop of vanilla ice cream. For an extra touch of indulgence, sprinkle additional oat crumble over the ice cream.