01 - Preheat oven to 350°F. Grease a 2-quart baking dish or an 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 - Pour the fruit mixture into the prepared baking dish, spreading it into an even layer.
04 - In a separate bowl, whisk together flour, oats, brown sugar, cinnamon, and salt. Add cold butter cubes and use a pastry cutter or your fingers to work the butter into the dry ingredients until coarse crumbs form.
05 - Sprinkle the oat crumble mixture evenly over the fruit filling, covering completely.
06 - Bake for 40 minutes until the topping is golden brown and the fruit filling is bubbling around the edges.
07 - Let the crisp cool for at least 15 minutes before serving to allow the filling to set. Serve warm with vanilla ice cream if desired.