Line a 9x12" baking dish with parchment paper. Chop the rhubarb in 1/4" pieces and add to baking dish along with sugar and 1/3 cup flour. Use your hand to toss everything together.
Make the topping:
In a separate bowl, stir together flour, sugar and salt. Use your fingers to work the butter into the dry ingredients until mixture resembles wet sand and no dry flour remains. Crumble topping evenly over the rhubarb. Bake for 35-40 minutes, until filling is bubbling and topping is golden.