Melt the butter and white chocolate in a pan over low heat.
Grind the walnuts roughly (I used a coffee grinder).
Sift the semolina, flour and bicarb into a bowl then stir in the walnuts.
Make a well in the middle and break in the eggs. Stir from the centre a little. Add the yogurt and stir a little further towards the edges. Add the butter and stir until all incorporated.
Grate in the lemon zest and stir once more.
Turn into a greased or lined 8" sq, cake pan and bake at 180C for 25 minutes or until an inserted skewer comes out clean.
Meanwhile dissolve the sugar in the water in a pan over a low heat. Then add the honey and lemon juice and simmer for about 10 minutes when the syrup should have thickened and reduced. Remove from the heat and add the rosewater.
Pour slowly over the hot cake making sure all is covered. It will seem like a lot of liquid, but the cake will absorb it all. Leave until cold, then turn out of the tin and cut into squares or diamonds.