Heat a baking dish in the oven (I used a heat proof glass one)
Melt 4 tablespoons sugar in a saucepan on the stove.
When the sugar begins to slightly caramelize, pour into the hot baking dish and move the dish so that the caramelized sugar sticks to the walls until it's spread completely and has a light amber color.
Let the caramelized sugar to cool until apples are prepared.
Preheat oven at 350 F
Clean and peel apples and pick the middle and seeds out with a small, sharp knife or an apple corer. Add the peeled apples over the caramelized sugar into the baking dish.
Sprinkle the other 2 tablespoons of sugar, cinnamon and walnuts.
Randomly place small pieces of butter
Drizzle lemon juice.
Move the dish to the oven and bake the apples for approx. 20- 25 min.
Make the batter while the apples are baking. In a bowl, cream egg yolks with sugar until the mixture is a light and fluffy. Whisk flour and baking powder and gradually add to the egg yolks.
Beat the egg whites until they're foamy and then mix with the rest of the batter by gently folding it in.
When the apples are soft, remove from oven. Pour the batter over and return the dish back to the oven.
Bake for additional 20-30 minutes, until the cake is done (you can check with a toothpick inserted in the batter).
Take the cake out of the oven and serve as is or place a large plate over the pot and flip it quickly so the apples will be on top