01 - Preheat the oven to 350°F. Lightly grease a 9-inch square or round baking dish.
02 - In a large bowl, toss the hulled strawberries and diced rhubarb with granulated sugar, cornstarch, vanilla extract, and lemon juice until evenly coated. Spread the mixture in a uniform layer across the bottom of the prepared baking dish.
03 - In a separate bowl, combine the rolled oats, all-purpose flour, packed brown sugar, ground 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 some pea-sized butter pieces remaining.
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 deeply golden brown.
06 - Allow the crisp to cool for 10 to 15 minutes before serving. Best enjoyed warm, optionally topped with vanilla ice cream or whipped cream.