First prepare the batter. In a medium mixing bowl rub sugar and lemon zest together to release oil. Add eggs and mix until creamy. Add oil and mix to combine. Add plain yogurt and vanilla extract and mix to incorporate.
Add the flour, salt and baking powder and mix to combine. Set aside until you prepare the apples.
Prepare the apples. Cut peeled and cored apples into ¼ inch (6mm) thick slices.
Place a 9 inch (23cm) frying pan over medium heat. Add butter and let it melt.
Add sugar and continue cooking until sugar is dissolved and gets a caramel color.
Remove from heat and arrange apple slices on top of the caramel. Sprinkle the cinnamon on top of the apples.
Pour the cake batter on top of the apples and place over low heat. Cover and cook for about 20 minutes or until a toothpick inserted into the center of the cake comes out clean.
Invert onto a serving platter.
Serve warm with ice cream or whipped cream on top.