Preheat your oven to 325°F (163°C) and prepare two 8x4-inch loaf pans by greasing with butter or vegetable oil, then dusting with flour.

In a large bowl, combine brown sugar, vegetable oil, beaten egg, buttermilk, and vanilla extract. Whisk until smooth.

In another bowl, whisk together all-purpose flour, baking soda, and salt. Gradually fold the dry ingredients into the wet mixture.

Toss rhubarb with a tablespoon of flour, then fold it and chopped walnuts or pecans into the batter.

Divide the batter between the two prepared loaf pans, filling them about two-thirds full.

Mix sugar and ground cinnamon in a small bowl. Sprinkle this over the tops of the batter, then drizzle with melted butter.

Bake for 50–60 minutes until a toothpick comes out clean or with few crumbs. The tops should be golden-brown.

Allow the bread to cool in the pans for about 10 minutes, then transfer to a wire rack to cool completely.