A great summery appetizer! We enjoyed a similar salad as a light lunch at the restaurant of a wine farm near Robertson the Western Cape Province, and ever since I've been imitating it. Of course small soft mozzarella balls (bocconcini) are preferable, as is home-made pesto, but let's face it, these real deli goodies are not always available. I used a good bottled pesto and ordinary mozzarella. You can use shallots in place of the small (pickling type) onions. This is great with a glass of chilled dry white wine and some still-warm home-made bread!