Trim the rhubarb and cut into 1-inch pieces, then place in a large bowl. Add the granulated sugar, 1/4 cup flour, cornstarch, vanilla, and freshly ground cardamom. Gently toss everything together until the rhubarb is evenly coated with the sugar mixture. The cornstarch and flour will help absorb the rhubarb's juices as it cooks, preventing a soupy filling. Let this sit while you prepare the crumble topping.

Add the 1 cup all-purpose flour, salt, brown sugar, and cold butter cubes to a food processor. Pulse until the mixture resembles coarse breadcrumbs with some pea-sized pieces of butter still visible—this ensures a tender, crumbly texture rather than a dense topping. Stir in the rolled oats by hand after pulsing to keep them from breaking down too much. I like to use really cold butter straight from the fridge and cut it into small cubes; it creates those beautiful irregular crumbles that bake up perfectly crispy.

Spread the rhubarb filling from Step 1 evenly into a 9x13-inch baking dish or similar size. Distribute the crumble topping from Step 2 over the rhubarb, breaking up any large clumps with your fingers so it covers the filling in an even but rustic layer. Bake at 375°F for 35-45 minutes, until the topping is golden brown and the rhubarb filling bubbles around the edges. The crumble is done when you see juices starting to emerge at the sides—this indicates the rhubarb has softened and released its juices.

Remove the crumble from the oven and let it cool for at least 30 minutes at room temperature. This resting time allows the filling to set slightly so it won't be runny when you serve it. I find that serving it slightly warm (not piping hot) with cold vanilla ice cream or whipped cream gives you the best contrast of temperatures and textures.