01 - Set the oven to 350°F (175°C) and allow it to fully preheat.
02 - In a large mixing bowl, blend granulated sugar with corn starch. Add sliced rhubarb and apple chunks, tossing gently to coat the fruit evenly. Transfer the mixture to a greased 8-inch square baking dish, spreading it into an even layer.
03 - In a separate bowl, combine old-fashioned oats, packed brown sugar, melted butter, ground cinnamon, and all-purpose flour. Stir until the mixture forms coarse crumbs.
04 - Distribute the prepared oat topping evenly over the fruit mixture. Place the baking dish on the center rack and bake for 50 minutes, or until the topping is golden brown and fruit is bubbling and tender.
05 - Allow the crisp to sit for 15–20 minutes to let the juices thicken. Serve warm or at room temperature.