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, toss the sliced strawberries and diced rhubarb with granulated sugar, cornstarch, vanilla extract, and lemon juice until evenly coated. Spread the fruit mixture in a single, even layer across the bottom of the prepared baking dish.
03 - In a separate bowl, combine the rolled oats, all-purpose flour, packed 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 resembles coarse crumbs with some pea-sized butter pieces remaining.
04 - Sprinkle the oat topping evenly over the fruit filling, covering the surface completely.
05 - Bake on the center rack for 35 to 40 minutes, until the fruit juices are bubbling around the edges and the topping is deep golden brown.
06 - Remove from the oven and let cool for 10 to 15 minutes before serving. Serve warm, optionally with a scoop of vanilla ice cream or a dollop of whipped cream.