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 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 into an even layer.
03 - In a separate 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 crumbs with some pea-sized pieces of butter remaining.
04 - Sprinkle the oat topping evenly over the fruit filling, covering the surface completely. Press down gently to help the topping adhere.
05 - Place the baking 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 is deep golden brown.
06 - Remove from the oven and let the crisp rest for 10 to 15 minutes before serving. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream.