OK, I admit that I have cheated very slightly here and toasted the sourdough; but toasting isn't exactly cooking, is it? The pesto will make far more than you actually need but I find that it does tend to keep very well for a few weeks if you store it in an airtight jar or container in the fridge.
4 x 1cm-thick slices from a small sourdough loaf
4 good-quality small buffalo mozzarellas which have been left at room temperature for a couple of hours
Flaky sea salt and freshly ground black pepper
A couple of tablespoons of extra virgin olive oil
A handful of small basil leaves, preferably Greek or bush basil
For the pesto
20g pine nuts, lightly toasted
50-60g fresh basil leaves and any soft stalks
2 cloves of garlic, peeled and crushed
A good pinch of sea salt
100-120ml extra virgin olive oil (preferably a sweeter olive oil)
6tbsp freshly grated Parmesan or Pecorino romano
To make the pesto, put the pine nuts, basil, garlic salt and olive oil in a liquidiser and coarsely blend. Add the cheese and blend again briefly.
Toast the sourdough on both sides and spread with a good spoonful of pesto, then place on serving plates.
Carefully tear the mozzarella in half and place on the sourdough. Season the mozzarella with sea salt and freshly ground black pepper and spoon the olive oil over and around and scatter the basil leaves all over.