Preheat oven to 325 degrees.
In a bowl mix butter, sugar and salt with a wooden spoon. Add flour and semolina. Mix well. The dough is crumbly, more like couscous.
Grease a tart pan and press down dough from the center to the edges.
Bake for 20 minutes.
Meanwhile mix egg yolks, lemon zest and juice of lemons and sugar in a heat-proof bowl. Put on low temperature and stir until mixture thickens, about 15 minutes. Remove from the heat and add in butter. Slice a lemon into thin slices.
When crust is ready, get it out of the oven and add custard. Put lemon slices onto custard. Bake for another 10 minutes. Serve after cooling completely.