To make the pesto, add the rocket, basil, garlic, and olive oil to a food processor, then blitz until smooth. Season to taste with salt and pepper.
Add the mozzarella to a bowl and pour over the pesto. Mix to coat. Leave to one side.
Remove the basil leaves from the stalks and chop any larger leaves.
Assemble your salads — dividing the rocket, watercress, basil, and tomatoes between your serving plates.
Drizzle over the balsamic vinegar and add the pesto-coated mozzarella.