Step 1: Preheat your oven to 375°F (190°C). Lightly grease an 8x8 inch or 9x9 inch baking dish.
Step 2: In a large bowl, combine the chopped rhubarb, granulated sugar, 1/4 cup all-purpose flour, and vanilla extract. Toss gently until the rhubarb is evenly coated.
Step 3: Pour the rhubarb mixture into the prepared baking dish, spreading it out evenly.
Step 4: In a separate medium bowl, combine the rolled oats, 1/2 cup all-purpose flour, light brown sugar, cinnamon (if using), and salt. Whisk to combine.
Step 5: Add the cold, cubed unsalted butter to the dry topping mixture. Using your fingertips or a pastry blender, cut the butter into the dry ingredients until the mixture resembles coarse crumbs with some pea-sized pieces of butter remaining.
Step 6: Evenly sprinkle the oat topping over the rhubarb mixture in the baking dish.
Step 7: Bake for 40-50 minutes, or until the rhubarb is tender and bubbling, and the topping is golden brown and crisp. If the topping browns too quickly, loosely tent with foil.
Step 8: Remove from the oven and let cool on a wire rack for at least 15-20 minutes before serving.
Step 9: Serve warm, optionally with vanilla ice cream or whipped cream. Enjoy this classic rhubarb crisp recipe!