Step 1: Preheat your oven to 375°F (190°C). Lightly butter a 9x13 inch baking dish.
Step 2: Prepare the Rhubarb Filling. In a large bowl, combine the chopped rhubarb, granulated sugar, all-purpose flour, and a pinch of cinnamon (if using). Toss gently until the rhubarb is evenly coated. The flour helps to thicken the juices released by the rhubarb during baking, preventing a watery filling.
Step 3: Make the Crumble Topping. In a separate medium bowl, whisk together the rolled oats, all-purpose flour, light brown sugar, and a pinch of salt.
Step 4: Cut in the Cold Butter. Add the cold, diced unsalted butter to the dry topping ingredients. Using your fingertips, a pastry blender, or two knives, cut the butter into the mixture until it resembles coarse crumbs with some pea-sized pieces remaining. Pro Tip: Working quickly keeps the butter cold, which is essential for a crisp topping. Avoid overworking the dough, as this can lead to a tough topping.
Step 5: Assemble the Crumble. Pour the rhubarb filling into the prepared baking dish, spreading it into an even layer. Evenly sprinkle the crumble topping over the rhubarb. Ensure a good, generous layer of topping for that perfect crunch.
Step 6: Bake to Golden Perfection. Place the baking dish on a baking sheet (to catch any potential overflow) and bake for 40-50 minutes, or until the topping is golden brown and the rhubarb filling is bubbling and tender when pierced with a fork. Pro Tip: If the topping starts to brown too quickly, you can lightly tent it with aluminum foil.
Step 7: Cool and Serve. Remove the crumble from the oven and let it cool for at least 15-20 minutes before serving. This allows the filling to set slightly and prevents it from being too runny. Serve warm, perhaps with a scoop of vanilla ice cream, a dollop of whipped cream, or a generous pour of custard. Enjoying it warm enhances the comforting flavors!