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 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 rolled oats, all-purpose flour, brown sugar, cinnamon, and salt. Add the cold cubed 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, 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 rest for 10 to 15 minutes before serving. Serve warm, optionally topped with a scoop of vanilla ice cream or a dollop of whipped cream.