Preheat the oven to 350 degrees F.
Place the 2 1/2 cups of chopped rhubarb in a small saucepan and cook, stirring occasionally, for about 15 minutes or until most of the water (but not all) has evaporated and the rhubarb is really soft. Set aside.
Prepare a standard loaf pan (8 1/2 inches by 4 1/2 inches) by spraying with cooking spray and coating with flour.
In a large bowl, whisk together the flour, baking powder, baking soda, salt and sugar. Add the walnuts, stir in and set aside.
In a smaller bowl, mix together the oil, cooked rhubarb and flax eggs.
Add the rhubarb-oil mixture to the dry ingredients and mix until combined.
Pour the batter into the prepared loaf pan. Cut the reserved stalk of rhubarb into slivers long enough to lay horizontally or vertically in the pan. Arrange them on top of the batter. Sprinkle on some sugar on the top.
Bake the bread in the preheated oven for 50-60 minutes with a toothpick. If the toothpick comes out clean when inserted into the center, your bread is ready.
Cool on a rack for 30 minutes, unmold from the pan and continue to cool.