Upside Down Rhubarb Cake

Directions

1. Preheat oven to 375 F. Over medium heat, add olive oil into a well-seasoned, 9-10 inch cast iron skillet, and brush the sides with it. Add rhubarb, ¾, cup sugar, lemon juice, 2 teaspoons balsamic, and a pinch of salt and stir for 2 minutes, just until combined and sugar has dissolved. Turn the heat off. Do not continue to cook rhubarb.
2. In a medium bowl, whisk egg yolks with lemon zest, cardamon, and ¼, cup sugar, until creamy and the sugar has dissolved. Set aside. In a small bowl, whisk the almond flour, salt, and baking powder until well combined. Add flour mixture to egg mixture and stir to incorporate. You will have a very thick heavy batter at this point.
3. In the bowl of a stand mixer whip egg whites on medium-high speed, and once you see bubbles, add a pinch of salt and the remaining teaspoon of vinegar. Whip until egg whites have formed soft peaks, then add the remaining ¼, cup sugar a little at a time whipping until medium-stiff peaks form. Turn speed to down to medium-low and begin spooning in the thick flour &ndash, egg yolk batter, a heaping spoonful at a time, into the egg whites, until it is all combined, scraping down sides as necessary.
4. Pour batter over rhubarb in the prepared cast iron skillet and place in a preheated oven, for 30 minutes. While baking, make sure to have ready, what you plan to flip the cake onto. At 30 minutes, check for doneness by inserting a toothpick or skewer into the middle of the cake. If it comes out clean, it is done. If not, bake 5 minutes longer, and check again. Pull from the oven, and if making in a skillet, immediately place a cake platter or cake stand over the skillet, and invert, gently tapping the bottom of the skillet so it releases. Serve with vanilla ice cream or on its own. The cake is especially good the next day.