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 uniformly across the bottom of the prepared baking dish.
03 - In a separate bowl, whisk together the rolled oats, all-purpose flour, light brown sugar, cinnamon, and salt. Add the cold diced butter and cut it in using a pastry cutter or your fingertips until the mixture forms coarse, pea-sized crumbs.
04 - Scatter the oat topping evenly over the fruit filling, ensuring full coverage across the surface.
05 - Place the dish in the center of the oven and bake for 35 to 40 minutes, until the fruit juices are bubbling around the edges and the topping is deeply golden brown.
06 - Remove from the oven and allow the crisp to rest for 10 to 15 minutes before serving. Serve warm, optionally accompanied by vanilla ice cream or freshly whipped cream.