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 hulled 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 cubed butter and using your fingertips or a pastry cutter, cut the butter into the dry ingredients 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. Gently press down to help the topping adhere.
05 - Place the dish in the center of the oven and bake for 35 to 40 minutes, or until the fruit juices are actively bubbling around the edges and the topping has turned a deep golden brown.
06 - Remove from the oven and allow the crisp to rest for 10 to 15 minutes before serving. This resting period allows the fruit juices to thicken slightly. Serve warm with a scoop of vanilla ice cream or a dollop of whipped cream if desired.