Step 1: Preheat your oven to 375°F (190°C). Lightly butter a 9x13 inch baking dish or an 8x8 inch square dish.
Step 2: In a large bowl, combine the chopped rhubarb, 1 cup granulated sugar, 1/4 cup flour (or cornstarch), and vanilla extract (if using). Toss gently until the rhubarb is evenly coated. Pour the mixture into the prepared baking dish and spread it into an even layer.
Step 3: In a separate medium bowl, prepare the crisp topping. Whisk together 1 cup flour, brown sugar, 1/4 cup granulated sugar, rolled oats, cinnamon (if using), and salt.
Step 4: Add the cold, cubed 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 of butter remaining. Do not overmix.
Step 5: Evenly sprinkle the crisp topping over the rhubarb mixture in the baking dish, ensuring it's well covered.
Step 6: Bake for 40-50 minutes, or until the rhubarb filling is bubbly around the edges and tender when pierced with a knife, and the topping is golden brown and crisp. If the topping starts to brown too quickly, you can loosely tent the dish with aluminum foil.
Step 7: Remove from the oven and let the rhubarb crisp cool for at least 15-20 minutes before serving. This allows the filling to set slightly. Serve warm with vanilla ice cream, whipped cream, or custard for the best experience.