Rhubarb is one of the first produce items to ripen in spring, making it a much sought-after commodity in Irish farmhouse kitchens. The crumble recipe works well with other fruits as well—just eliminate the cornstarch when you try it with peaches, plums or apples.
Preheat oven to 400F. Coat 9×13-inch baking dish with cooking spray.
To make Rhubarb Filling: Toss together all ingredients in glass or other nonreactive bowl. Let stand 15 minutes to release juices.
To make Oatmeal Topping: Place all ingredients in bowl of food processor and pulse until mixture is chunky and begins to hold together.
Spoon rhubarb mixture into prepared baking dish with slotted spoon, leaving any liquid in bowl. Discard liquid. Top with oatmeal mixture. Bake 35 to 40 minutes, until top is lightly browned. Cool on rack 10 minutes before serving.