2 lbs rhubarb, sliced crosswise 3/4 inch thick
1 cup granulated sugar
3/4 cup all-purpose flour
1/2 cup cold unsalted butter, cut into pieces
1/2 cup packed light-brown sugar
1 cup rolled oats
1/2 teaspoon ground cinnamon
vanilla ice cream, for serving (optional)

Preheat oven to 400°. In a 9-by-13-inch baking dish, combine the rhubarb, 1 cup sugar, and 1/4 cup flour; set aside. In the bowl of a food processor, combine remaining 1/2 cup flour and the butter. Pulse until the butter pieces are the size of coarse oats. Add brown sugar, oatmeal, and cinnamon. Pulse to combine. Sprinkle over rhubarb. Bake until rhubarb is tender and topping is golden, 35 to 45 minutes. Serve warm with ice cream, if desired.