Combine the remaining 1 cup of flour, the oats, and ¹/₈ teaspoon of salt in a large bowl. Using the large holes on a box grater, grate the cold butter into the bowl. Mix with your hands or a pastry cutter until the crumble mixture is evenly combined and sticks together. Add the remaining ¼ cup of granulated sugar, the brown sugar, and 1½ teaspoons of the lime juice and continue mixing with your hands until the mixture comes together in one mass. Now crumble it over the rhubarb mixture, making sure to cover the fruit evenly and completely.