01 - Preheat 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. Add granulated sugar, cornstarch, vanilla extract, and lemon juice. Toss gently until all the fruit is evenly coated, then spread the mixture in an even layer across the bottom of the prepared baking dish.
03 - In a separate bowl, whisk together the rolled oats, all-purpose flour, brown sugar, cinnamon, and salt. Add the cold cubed butter and use your fingertips or a pastry cutter to cut it into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
04 - Sprinkle the oat topping evenly over the fruit filling, covering the surface completely.
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 let the crisp rest for 10 to 15 minutes before serving. The filling will thicken as it cools. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream if desired.