01 - Preheat the oven to 350°F. Lightly grease a 9-inch square or round baking dish with butter or nonstick spray.
02 - In a large mixing bowl, toss the hulled strawberries and diced rhubarb with granulated sugar, cornstarch, vanilla extract, and lemon juice until evenly coated. Spread the fruit mixture in an even layer across the bottom of the prepared baking dish.
03 - In a separate bowl, combine the rolled oats, all-purpose flour, packed brown sugar, cinnamon, and salt. Add the cold diced butter and cut it in using your fingers or a pastry cutter until the mixture resembles coarse crumbs with pea-sized pieces of butter throughout.
04 - Sprinkle the oat topping evenly over the fruit filling, covering the surface completely.
05 - Bake for 35 to 40 minutes, or until the fruit juices are bubbling around the edges and the topping is golden brown and crisp.
06 - Remove from the oven and allow to cool for 10 to 15 minutes before serving. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream if desired.