Preheat oven to 350°F. Lightly spray or butter a 7-inch x 11-inch or 8.5-inch x 11-inch baking dish and set aside.
Mix together the flour, oats, cinnamon, brown sugar, and melted butter.
Press one-half of the crumb mixture into the prepared baking dish.
Add chopped rhubarb atop the pressed-in crumb mixture in the baking dish.
Whisk sugar, cornstarch, and water together in a glass bowl. Microwave until thick, clear, and bubbly - approximately 3-5 minutes. Remove from the microwave and whisk in the vanilla. Pour the sugar sauce evenly over rhubarb.
Top with evenly with the remaining crumb mixture.
Bake until edges are bubbling and the crumb topping is browned, about 30-40 minutes.
Cool slightly before serving.
Notes
🥡 STORAGE
Store leftovers covered in the refrigerator for up to 4 days.
♨️ REHEATING
Reheat individual portions in the microwave for 20-30 seconds.
Warm in a 300°F oven until heated through.
The air fryer is not recommended, as the topping may overcook.
Reheating in a skillet is not ideal for this dessert.
❄️ FREEZING
Cool completely before wrapping tightly and freezing. Freeze for up to 3 months. Thaw in the refrigerator before reheating.
💡 TIPS FOR BEST RESULTS
If using frozen rhubarb, thaw it first and then drain.
For a thicker sauce, increase the cornstarch to 2-½ tablespoons.
Serve warm with whipped cream or vanilla ice cream.
Let cool slightly before serving to allow the filling to set.
🍳 ALTERNATE COOKING METHODS
The oven is the only recommended method for this recipe.
Not suitable for air fryer, microwave baking, or stovetop methods.
♻️ LEFTOVERS
Use leftover rhubarb crisp as a topping for yogurt, pancakes, or oatmeal.
Tastes great chilled, at room temperature, or gently rewarmed.