In a large bowl, toss together the rhubarb and ginger. Transfer to a 2 or 3 quart baking dish and sprinkle 2 cups of the sugar over top of the rhubarb mixture; set aside.
In a medium bowl, whisk together the flour, remaining sugar and salt. Add the butter and use a pastry blender (or two knives or your fingers) to cut the butter into the flour mixture until it resembles coarse meal and the butter pieces are no larger than the size of a pea. Sprinkle the crumble topping evenly over the rhubarb.
Bake until the topping is golden brown, the rhubarb mixture is bubbling and the rhubarb is soft, about 1 hour 15 minutes. Place the pan on a wire rack to cool slightly; the crumble can be served warm or at room temperature. Top with ice cream or fresh whipped cream, if desired.