Preheat the oven to very hot (240 c, 475 f, Gas Mark 9).
Remove the Mozzarella from its packet of preserving liquid, wipe dry and slice.
Remove the crusts from the bread and cut each slice in half. Peel and crush the garlic cloves.
Wash, drain and finely chop the parsley. Blend together the anchovy fillets, capers and garlic in a liquidiser, or pass through a sieve to make a smooth paste.
Stir 4 tablespoons of olive oil into the paste, followed by the chopped parsley. Season with pepper. Spread half the bread slices with the paste, top with the cheese and cover with another slice of bread.
Sprinkle both sides of the crostini with the remaining oil, place on a baking sheet and bake for a few minutes in the preheated oven until golden brown. Turn once during baking.
Remove the crostini from the oven, leave to cool slightly and serve with black olives.