Put vinegar in a medium saucepan and bring to a boil over medium heat. Adjust heat to low and simmer until it is reduced nearly three-quarters in volume and thickened to a syrup. About 15 minutes.
Add plums to syrup turning gently to coat and continue to cook for another 4-7 minutes more. Turn off heat and let cool.
Meanwhile assemble mozzarella slices on platter, allowing to overlap slightly and season with salt and pepper to taste. With a fork, remove plums from syrup and place atop cheese, then top with the remaining syrup, pine nuts and mint. Drizzle with olive oil and serve.
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