Preheat oven to 350°F (175°C) and grease a 9x5-inch loaf pan.
Stir milk and lemon juice together; set aside to curdle slightly.
In a large bowl, whisk brown sugar, oil, egg, and vanilla.
Add the milk mixture and stir until smooth.
Mix in flour, baking soda, salt, and cinnamon until just combined.
Fold in chopped rhubarb and walnuts gently.
Pour batter into the greased loaf pan and smooth the top.
Bake for 50–55 minutes, or until a toothpick comes out clean.