Preheat oven to 325 degrees F (165 degrees C). Lightly grease two 9x5 inch loaf pans. In a small bowl, stir together milk, lemon juice and vanilla; let stand for 10 minutes.
In a large bowl, mix together 1 1/2 cups brown sugar, oil and egg. Combine the flour, salt and baking soda, stir into sugar mixture alternately with the milk mixture just until combined. Fold in rhubarb and nuts. Pour batter into prepared loaf pans.
In a small bowl, combine 1/4 cup brown sugar, cinnamon and butter. Sprinkle this mixture over the unbaked loaves.
Bake in preheated oven for 40 minutes, until a toothpick inserted into center of a loaf comes out clean.
Letting the milk and lemon juice stand for exactly ten minutes before mixing creates a soured dairy base. This acidic addition reacts directly with the baking soda to generate carbon dioxide gas, providing the primary leavening lift for dense batter containing heavy vegetables and nuts. Combining wet and dry components requires restraint. Overmixing activates the gluten network within the all-purpose flour, yielding a tough, rubbery loaf rather than a tender crumb. Stir just until the dry pockets disappear before folding in the chopped fruit and walnuts. Acidic ingredients also tame the natural sharpness of raw rhubarb stalks while softening their fibrous structure during the 40-minute bake at 325 degrees F. Standard loaf pans measuring 9x5 inches ensure the dense batter bakes evenly to the center without burning the sugary cinnamon topping. Pair slices of this spiced bread alongside savory breakfast proteins like crisp bacon or soft-scrambled eggs. The natural salinity and fat content of meat or dairy cut through the sweet brown sugar crust and highlight the bright, tart notes of the baked vegetable. Store cooled loaves wrapped tightly in plastic wrap at room temperature for up to three days, or freeze individual slices for longer preservation. Substitute pecans or hazelnuts for the walnuts if desired, maintaining the same half-cup volume to preserve the structural integrity of the batter.