01 - Preheat oven to 350°F. Lightly grease a 9-inch square baking dish.
02 - Combine rhubarb, strawberries, granulated sugar, cornstarch, vanilla extract, lemon juice, and salt in a large bowl. Toss thoroughly to coat, then spread evenly in the prepared baking dish.
03 - Mix oats, flour, brown sugar, granulated sugar, cinnamon, and salt in a separate bowl. Add cold butter cubes and use fingers or a pastry blender to work butter into dry ingredients until mixture resembles coarse crumbs.
04 - Sprinkle the oat crumble evenly over the fruit filling.
05 - Bake for 40 minutes until topping is golden brown and fruit bubbles around the edges.
06 - Cool for at least 15 minutes before serving. Serve warm or at room temperature with vanilla ice cream or whipped cream.