In a saucepan, combine the sugar, cornstarch, and salt. Add rhubarb and strawberries, toss to coat. Let stand for 5 minutes. Bring to a boil, cook and stir for 1 minute. Pour into a greased 8 inch square baking dish.
In a bowl, combine the flour, sugar, baking powder, and salt, cut butter until mixture resembles coarse crumbs. Combine egg and milk, stir into crumb mixture just until moistened. Drop by tablespoonfuls onto fruit. Sprinkle with additional sugar.
Bake at 400 degrees for 20-25 minutes or until filling is bubbly and topping is golden brown. Cool for 10 minutes before serving.