Blanch the broad beans or peas in boiling water until tender. Drain and season, then place in a bowl with the olives and rocket. Toss to combine.
Make the dressing. Put the mustard and sherry vinegar into a bowl and season well. Gradually add the olive oil, whisking to emulsify. Squeeze in the lemon juice and whisk to combine. Check seasoning.
Tip the salad onto a serving plate. Tear each mozzarella ball into four pieces and place on top. Drizzle with the dressing and scatter over the lemon zest.