Gather the ingredients. Preheat the oven to 325 degrees F (165 degrees C). Lightly grease two 9x5-inch loaf pans.

Stir milk, lemon juice, and vanilla together in a small bowl; let stand for 10 minutes.

Mix 1 ½ cups brown sugar, oil, and egg in a large bowl. Combine flour, salt, and baking soda; stir into sugar mixture alternately with milk mixture just until combined. Fold in rhubarb and walnuts.

Pour batter into prepared loaf pans. Combine remaining 1/4 cup brown sugar, butter, and cinnamon in a small bowl. Sprinkle this mixture over the unbaked loaves.

Bake in the preheated oven until a toothpick inserted into the center of a loaf comes out clean, about 40 to 60 minutes.