01 - Preheat oven to 350°F. Lightly grease an 8-inch square baking dish with butter or nonstick spray.
02 - In a large mixing bowl, toss the hulled strawberries and sliced rhubarb with granulated sugar, cornstarch, vanilla extract, and lemon juice until the fruit is evenly coated. Transfer the mixture to the prepared baking dish, spreading it into an even layer.
03 - In a separate bowl, whisk together the rolled oats, all-purpose flour, packed brown sugar, cinnamon, and salt. Add the cold cubed butter and use your fingers or a pastry cutter to cut it into the dry ingredients until the mixture resembles coarse, pea-sized crumbs.
04 - Sprinkle the oat topping evenly over the fruit filling. Bake for 35 to 40 minutes, until the topping is deep golden brown and the fruit juices are bubbling around the edges.
05 - Allow the crisp to rest for 10 to 15 minutes before serving so the filling can set slightly. Serve warm, with a scoop of vanilla ice cream or a dollop of whipped cream if desired.