Bring water to a boil with the bouillon cube, then add the polenta and stir
Reduce the heat to low and stir frequently during around 20 min
During this time you can also prepare the other ingredients; chop the tomatoes, crush the walnuts and cut the cheese in small cubes
Remove pot from the heat stir in all the remaining ingredients
Pour some cold water on an oven sheet and then put the polenta on it. Press it out to around 1-2 cm thickness. Let it cool for an hour
Just before serving, brush it with oil and grill on high heat in the oven for around 10 min
Cut the cake in smaller pieces and serve on a plate with your choice of lettuce