Preheat oven to 325 degrees F. Grease and flour an 8x4 inch (6 cup capacity) loaf pan
Whisk the flour, salt and baking soda together in a bowl and set aside
In a mixing bowl, whisk together the 2 tablespoons of egg, brown sugar, oil, and vanilla.
Add the flour mixture and buttermilk alternately to the sugar mixture, stirring just until combined.
Stir in rhubarb and orange, then spread the batter in the prepared loaf pan.
Topping: In a bowl, combine the flour, sugars and cinnamon. Add the butter and stir until crumbly. Sprinkle over top of the loaf.
Bake for for 45-55 minutes, until a toothpick inserted into center of a loaf comes out clean or a meat thermometer registers 210 degrees. Let the loaf cool for about 40 minutes in the pan, then carefully remove from the pan and let cool completely. Serve when cool, or wrap tightly in plastic wrap and freeze until ready to serve.