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 bowl, toss the sliced 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, brown sugar, cinnamon, and salt. Add the cold diced butter and use your fingers or a pastry cutter to cut it in until the mixture resembles coarse crumbs with pea-sized pieces.
04 - Sprinkle the oat topping evenly over the fruit filling, covering the surface completely.
05 - Bake for 35 to 40 minutes until the fruit juices are bubbling around the edges and the topping is golden brown.
06 - Allow the crisp to cool for 10 to 15 minutes before serving. Serve warm with vanilla ice cream or whipped cream if desired.