01 - Preheat oven to 350°F.
02 - In a large bowl, combine rhubarb, strawberries, sugar, cornstarch, lemon juice, and vanilla. Toss gently until fruit is well coated.
03 - Transfer the fruit mixture to a lightly greased 8x8-inch baking dish.
04 - In another bowl, mix oats, flour, brown sugar, cinnamon, and salt. Add the cold butter cubes and use your fingers or a pastry cutter to blend until the mixture resembles coarse crumbs.
05 - Evenly sprinkle the oat crumble topping over the fruit. Bake for 35-40 minutes, or until the topping is golden brown and the fruit is bubbling around the edges.
06 - Let cool for at least 10 minutes before serving. Serve warm, optionally with vanilla ice cream.