Preheat oven to 350 degrees F (175 degrees C). Grease a 9-inch fluted tube cake pan.
Mix 1/4 teaspoon cinnamon with 2 teaspoons sugar in a small bowl. Pour mixture into greased cake pan and tilt the pan, tapping to coat inside of pan with cinnamon sugar. Pour out any loose cinnamon sugar.
Mix lemon juice into milk in a large measuring cup and let stand about 5 minutes to sour. Mix shortening and 1 1/2 cups sugar together in a large bowl until smooth; beat in egg and vanilla extract. Whisk flour, baking soda, and salt in a separate bowl. Gradually add dry ingredients to moist ingredients, alternating with sour milk, to make a smooth batter.
Stir rhubarb into batter. Pour half the batter into prepared cake pan. Stir brown sugar, 1/3 cup white sugar, 1 teaspoon cinnamon, and walnuts together in a bowl; sprinkle nut mixture over batter. Pour remaining batter over nut mixture.
Bake in the preheated oven until a toothpick inserted into the center of the cake comes out clean, 45 minutes to 1 hour. Cool in pan for 10 minutes before removing to a serving platter. Stir 2 teaspoons white sugar with 1/4 teaspoon cinnamon and sprinkle sugar mixture on top of cake.