Preheat the oven to 350°. Grease or line a loaf pan with parchment.
Cut the rhubarb to the size of the pan. It should cover the surface area completely. Next, sprinkle the sugar evenly to coat the bottom of the loaf pan. Line the pan with the rhubarb, with the flat side down.
Make the cake. In a medium-sized bowl, mix together the flours, baking powder, baking soda, and salt. Set aside.
Using a mixer, cream together the butter and sugar. Mix until the butter and sugar look fluffy, about 3-4 minutes. Scrape down the sides of the bowl. Add in the egg and egg yolk, one at a time making sure it is fully incorporated into the batter.
Add the dry ingredients into the butter mixture. Start the mixer off slow so the flour does not go flying!
Once the dry ingredients are incorporated, turn the mixture to low and slowly add in the milk.
Using a spatula, fold in the lemon juice and zest. Try not to overwork the batter. The batter will be a little bit thick, so don't worry if it looks it should be mixed more.
Pour batter on top of rhubarb and let bake for 35-45 min. The cake will look golden brown on the top when complete.
Once the cake is done, let it cool for about 20 minutes! Don't get tempted to flip it earlier, you need to let the rhubarb get comfortable and settle in.