Heat the oven. Preheat to 375°F (190°C). Lightly butter a 9x9-inch baking dish or similar 2-quart casserole.
Prep the rhubarb. Rinse and trim the stalks.Slice into even pieces so they cook at the same rate. If using frozen rhubarb, break up any clumps.
Mix the filling. In a large bowl, toss rhubarb with granulated sugar, cornstarch, zest (if using), and vanilla. Stir until the sugar and starch coat the fruit.Pour into the baking dish and spread evenly.
Make the crumble. In another bowl, combine flour, oats, brown sugar, salt, and cinnamon. Add the cold butter cubes and use your fingertips to rub them into the dry ingredients until the mixture forms pea-sized clumps. You want a mix of fine crumbs and larger bits for the best texture.
Top and bake. Sprinkle the crumble evenly over the rhubarb.Don’t press it down—those airy pockets help it stay crisp. Bake for 35–45 minutes, until the topping is golden and the filling is bubbling up around the edges.
Rest before serving. Let the crumble sit for at least 15 minutes. This gives the juices time to thicken so you get neat spoonfuls instead of a soupy mess.
Serve warm. Spoon into bowls and top with ice cream or a dollop of whipped cream.The contrast of warm fruit and cool cream is unbeatable.
