Line a 8 x 4 inch loaf pan with parchment paper and set aside.
Whisk the dry ingredients (flour, sugar, baking soda and baking powder) together in a large bowl.
In a separate bowl, combine the beaten egg, orange juice, orange rind and vegetable oil; add to the flour mixture.
Sprinkle with rhubarb and walnuts (if using) and stir until the composition is moist, but still lumpy.
Scrape into prepared pan.
Bake in the center of a preheated 350 degrees F oven for about 1 hour (until a tester inserted into the center comes out clean).
Leave the bread in the pan on a rack for about 10 minutes.
Remove the bread from the pan and let cool.
Serve within 24 hours.