1 pound rhubarb
3/4 cup sugar, divided
1/4 cup plus 2 tablespoons flour
1/4 cup (1/2 stick) unsalted butter, chilled, cut into pieces
1/2 cup rolled oats
1/4 teaspoon ground cinnamon
ice cream, for serving (strawberry or vanilla)
Preheat oven to 400 degrees. Slice the rhubarb stalks on the diagonal into uniform 3/4-inch-thick pieces. Place in a 2-quart baking dish; toss with 1/2 cup sugar and 2 tablespoons flour.
In a food processor, pulse the remaining 1/4 cup flour with the butter until the clumps are pea-size. Add the remaining 1/4 cup sugar, the rolled oats, and cinnamon; pulse just to combine. Sprinkle over the rhubarb.
Bake until the rhubarb is tender and the topping is golden, 35 minutes. Serve warm with ice cream.