Picture this: It's a hot summer afternoon, you're sitting at a rustic wooden table under the shade of a leafy olive tree. The sun is high, the air is filled with the seductive scent of the sea and grilling meat. You're served a bowl of something creamy, refreshing, subtly garlicky, and flecked with chopped green herbs. This could either be a bowl of Turkish cacık or Greek tzatziki, depending on which side of the Aegean Sea you're on.