01 - Preheat the oven to 350°F. Lightly grease a 9-inch square or round baking dish with butter or nonstick cooking spray.
02 - In a large mixing bowl, combine the sliced strawberries and diced rhubarb. Add the granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss gently until the fruit is evenly coated and the mixture is well combined. Transfer the filling to the prepared baking dish and spread it into an even layer.
03 - In a separate medium bowl, whisk together 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 into the dry ingredients until the mixture resembles coarse, pea-sized crumbs.
04 - Sprinkle the oat topping evenly over the fruit filling, ensuring complete coverage across the entire surface of the dish.
05 - Place the dish on the center rack of the oven and 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 the crisp to cool for 10 to 15 minutes before serving. Serve warm, optionally accompanied by vanilla ice cream or freshly whipped cream.