In a saucepan, combine sugar and water. Add the lemon slices. Bring to a boil and simmer over low heat for 5-7 minutes until slightly thickened. Add the lemon juice and remove from heat.
Allow the syrup to cool completely. It must be cold before pouring over the hot cake.
Preheat the oven to 170 °C (338 °F). Grease a small baking pan (approx. 20x30 cm) with butter and dust with flour/semolina.
Whisk the eggs with sugar using an electric mixer for about 5 minutes until a light, fluffy foam is formed.
Add the yogurt, oil, and lemon zest and whisk briefly on low speed.
In another bowl, mix the semolina, flour, and baking powder. Gently fold the dry ingredients into the wet ingredients using a spatula (not the mixer) until just combined.
Pour the batter into the prepared baking pan and bake in the preheated oven for 25-35 minutes, or until the cake is golden brown and a toothpick test comes out clean.
Remove the cake from the oven and immediately cut it into squares or diamonds.
Using a ladle, slowly and evenly pour the cold syrup over the hot cake. It will take a moment for the cake to absorb the syrup.
Let the cake stand at room temperature for at least 1-2 hours, ideally longer, for the syrup to fully penetrate. Sprinkle with coconut or ground pistachios before serving.