It's best to make the syrup first so that it has time to cool. Mix the water and sugar and let them come to a boil. Boil the syrup for 5 min. on moderate heat, add the lemon, cut into slices, and boil another 3 min. Remove from the stove and leave it to cool.
Beat the eggs with the sugar until you get a nice foam. Add the yoghurt, in which you've extinguished the baking soda.
Pour the prepared mixture in an oiled and floured oven dish and bake in a preheated 392°F (200 °C) oven until golden.
Take the sponge cake out of the oven and while still hot, pour the cooled sugar syrup over it. Garnish as desired and enjoy!