Crumble Topping: In a bowl, whisk flour with sugar. With pastry blender or 2 knives, cut in butter until mixture is in fine crumbs. Set aside.
In another bowl, whisk together sugar, flour and salt; whisk in egg until smooth. Add rhubarb and strawberries; toss to coat. Scrape into pie shell, then dot with butter. Sprinkle crumble topping evenly over filling. Bake in oven at 425 for 10 minutes. Reduce heat to 350 degrees and bake until topping is golden and filling is bubbly, approximately 40 minutes. Let pie cool on rack.