01 - Set oven to 350°F to ensure proper baking temperature.
02 - In a large bowl, combine rhubarb, strawberries, granulated sugar, cornstarch, lemon juice, and vanilla extract. Toss gently until fruit pieces are evenly coated with the sugar mixture.
03 - Pour the coated fruit mixture into a lightly greased 8x8-inch baking dish, spreading it into an even layer.
04 - In a separate bowl, whisk together rolled oats, flour, brown sugar, cinnamon, and salt. Add cold butter cubes and blend using your fingers or a pastry cutter until the mixture resembles coarse crumbs with some pea-sized butter pieces remaining.
05 - Sprinkle the oat crumble mixture evenly over the fruit filling, covering completely.
06 - Bake for 35-40 minutes until the topping is golden brown and the fruit filling is bubbling vigorously around the edges of the dish.
07 - Allow the crisp to cool for at least 10 minutes before serving. Serve warm, optionally topped with vanilla ice cream.