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, combine the sliced strawberries and diced rhubarb with granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss gently until the fruit is evenly coated, then spread the mixture in a uniform layer across the bottom of the prepared baking dish.
03 - In a separate bowl, whisk together the rolled oats, all-purpose flour, light brown sugar, ground cinnamon, and salt. Add the cold diced butter and cut it in using a pastry cutter or your fingertips until the mixture forms pea-sized crumbs with no dry flour remaining.
04 - Scatter the oat topping evenly over the fruit filling, covering the entire surface. Avoid pressing the topping down to maintain a light, crisp texture.
05 - Place the dish on the center rack and bake for 35 to 40 minutes, or until the fruit juices are bubbling around the edges and the topping has turned a deep golden brown.
06 - Remove from the oven and let the crisp rest for 10 to 15 minutes before serving. This allows the fruit filling to set slightly. Serve warm with a scoop of vanilla ice cream or a dollop of freshly whipped cream.