Preheat your oven to 375°F (190°C). Lightly butter a 9x9-inch baking dish and set aside.
Prepare the rhubarb. Wash the stalks thoroughly and trim away all leaves — they are toxic and must be discarded entirely. For thicker stalks, use a paring knife to strip away any tough fibrous strings running along the outside, similar to stringing celery. Cut the cleaned stalks into even ½-inch pieces until you have 4 full cups.
Make the filling. Add the rhubarb pieces to a large bowl. Add the granulated sugar, cornstarch, lemon juice, lemon zest, cinnamon, and vanilla extract. Toss everything together until the rhubarb is evenly and thoroughly coated. The cornstarch will look chalky at first — this is normal and it will dissolve into the juices during baking. Pour the filling into your prepared baking dish and spread into an even layer.
Make the crisp topping. In a medium bowl, combine the rolled oats, flour, brown sugar, cinnamon, and salt. Stir briefly to combine. Add the cold butter cubes. Using your fingertips, press and smear the butter into the dry ingredients until the mixture forms coarse, shaggy crumbles with irregular chunks — some fine, some pea-sized. Stop before it looks uniform. The irregular texture is what gives you that signature combination of crunch and chew after baking. Do not overmix.
Assemble the crisp. Scatter the oat topping loosely and evenly over the rhubarb filling, covering all the way to the edges of the dish. Do not press the topping down — leave it loose so hot air can circulate and crisp it properly.
Bake on the center oven rack for 40–45 minutes. The crisp is done when the topping is a deep, nutty golden brown and the filling is bubbling thickly around the edges and up through any gaps in the topping. Thin, watery bubbles mean it needs more time — wait for thick, jammy bubbles. If the topping browns too quickly before the filling bubbles, tent loosely with aluminum foil and continue baking for another 10 minutes.
Rest and serve. Remove from the oven and allow the crisp to rest for at least 15–20 minutes before serving. The filling continues to thicken as it cools — cutting in too early will result in a runny filling. Serve warm with a generous scoop of homemade vanilla ice cream or a dollop of crème fraîche.
