Combine oats, 1/2 cup flour, and brown sugar. Cut in butter until mixture is crumbly. Place half of the mixture in a greased 8-inch baking pan.
In a separate bowl, toss rhubarb with 1 tablespoon flour, 1/2 cup sugar, 1 teaspoon cinnamon, salt, and water. Spoon over first crumb layer in baking pan. Top with remaining crumbs. Bake at 350° for 35 to 45 minutes, until rhubarb is tender and topping is browned.
Serve rhubarb crunch warm, cut into squares and topped with vanilla ice cream or sweetened whipped cream.
Calories per serving: 306
Fat per serving: 12.4g