Make Rhubarb Crisp: a tangy, buttery dessert with a crunchy oat topping. Bake and serve warm with ice cream for seasonal comfort.
Preheat the oven to 375°F so it's ready by the time the crisp is assembled. This step makes sure the filling bubbles and the topping browns evenly; set a timer and line up your baking dish nearby so you can slide the crisp straight in when it's ready.
Rinse the rhubarb thoroughly to remove dirt, discard any leaves, and pat the stalks dry. Trim and cut into roughly 1/2-inch pieces (if the stalks are very thin, cut them into 3/4-inch segments so they hold some texture when baked). Keep the pieces fairly uniform so they cook at the same rate and present as pretty pink-red chunks when they soften.
In a medium bowl toss the cut rhubarb with 2/3 cup granulated sugar, 3 tablespoons all-purpose flour, and 1/2 teaspoon cinnamon until every piece is lightly coated and glossy with the sugar-flour mix. Scrape the coated rhubarb into a rectangular baking dish so the fruit sits level and the juices will gather evenly during baking.
In another medium bowl combine 3/4 cup rolled oats, 3/4 cup packed brown sugar, 1/3 cup flour, 1/3 cup unsalted butter and 1/2 teaspoon cinnamon (fold in 1/3 cup coconut flakes or chopped nuts if using). Work with a fork or pastry cutter until the mixture comes together in moist, clingy crumbs and a few larger clusters remain. Evenly sprinkle the topping over the rhubarb so there are pockets of oat clusters and buttery shards across the surface.

Slide the assembled rectangular dish into the preheated oven and bake for about 30–35 minutes, watching for a bubbly, syrupy filling and a deeply golden-brown, slightly crunchy topping. You'll know it's done when the center bubbles through the topping in a few glossy spots and the oats have toasted to a warm amber.
Remove the dish and let it cool on the counter for at least 10 minutes so the juices thicken slightly and the topping firms. This resting time makes scooping cleaner and lets the tart-sweet juices settle into syrupy ribbons around the fruit.
Spoon generous portions from the rectangular baking dish onto plates or serve directly from the dish; top each warm serving with a scoop of creamy vanilla ice cream or a drizzle of cream so it melts into the hot filling. Enjoy the contrast of tender, pink rhubarb and crunchy oat clusters with spoonfuls of melting ice cream.
