01 - Preheat oven to 350°F. Grease a 2-quart baking dish or 8-inch square pan with butter or cooking spray.
02 - In a large bowl, combine strawberries, rhubarb, granulated sugar, cornstarch, vanilla extract, lemon juice, and a pinch of salt. Toss gently until fruit is evenly coated.
03 - Transfer the fruit mixture to the prepared baking dish, spreading into an even layer.
04 - In a separate bowl, whisk together flour, oats, brown sugar, cinnamon, and salt. Add cold butter cubes and cut in using a pastry cutter or rub in with fingertips until mixture resembles coarse crumbs with some pea-sized pieces remaining.
05 - Sprinkle the oat crumble evenly over the fruit filling, covering completely.
06 - Bake for 40 minutes until topping is golden brown and fruit filling is bubbling around the edges. If topping browns too quickly, tent loosely with foil.
07 - Let cool for at least 15 minutes to allow filling to set. Serve warm, optionally topped with vanilla ice cream.