Preheat Oven:
Preheat oven to 350°F (175°C). Grease or line a 9x5-inch loaf pan with parchment paper.
Mix Wet Ingredients:
In a large bowl, whisk together the brown sugar, vegetable oil, egg, vanilla extract, and buttermilk until smooth.
Add Dry Ingredients:
In another bowl, whisk together flour, baking soda, and salt. Gradually add the dry mixture to the wet ingredients, stirring just until combined.
Fold in Rhubarb and Nuts:
Toss rhubarb with a teaspoon of flour to prevent sinking. Gently fold rhubarb and optional walnuts into the batter.
Prepare Topping and Assemble:
Pour the batter into the prepared pan and smooth the top. Mix granulated sugar and cinnamon, then sprinkle evenly over the batter.
Bake:
Bake for 55–65 minutes or until a toothpick inserted into the center comes out clean. Tent with foil after 40 minutes if the top browns too quickly.
Cool and Serve:
Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely. Slice and enjoy!