Insalata caprese—the salad of Capri—is the most famous Italian salad in the world and simultaneously the most abused, its perfection in Campania and its degradation elsewhere serving as a litmus test for the principle that great cooking requires great ingredients, not great technique. The canonical caprese consists of exactly four elements: ripe San Marzano or cuore di bue (ox-heart) tomatoes, mozzarella di bufala campana DOP, fresh basil leaves, and extra-virgin olive oil—with a scatter of sea salt. That is all. No balsamic vinegar (an abomination on caprese, according to every Campanian cook), no dried oregano, no pepper, no lettuce, no onion. The tomatoes must be at the peak of summer ripeness—warm from the sun, heavy with juice, their flesh sweet and acidic in equal measure. The mozzarella must be genuine buffalo mozzarella, torn or sliced thickly, at room temperature so its complex, tangy flavour is fully expressed. The basil must be fresh, ideally the small-leafed, intensely fragrant Genovese variety, whole leaves laid atop the arrangement rather than chiffonaded into oblivion. The olive oil should be a good Campanian or Southern Italian pressing, grassy and peppery. The assembly is artless: alternating slices or chunks of tomato and mozzarella, basil leaves scattered over, olive oil drizzled, salt flicked from the fingers. The salad represents a fundamental truth of Italian cooking philosophy: when ingredients are perfect, the cook's role is to step aside and allow them to speak. Outside of summer, and outside the geographic range where genuine buffalo mozzarella and ripe Mediterranean tomatoes are available, attempting a caprese is an exercise in futility that produces the pallid hotel-buffet travesty that has made the salad a cliché.
Use only peak-summer ripe tomatoes. Genuine buffalo mozzarella at room temperature. Fresh basil leaves, whole. Extra-virgin olive oil and sea salt only. No vinegar, no pepper, no additions. Don't attempt outside of summer.
Remove the mozzarella from the fridge 30+ minutes before serving. Use flaky sea salt, not fine table salt. Let the assembled salad sit for 5 minutes before serving—the tomato juices, mozzarella whey, and oil mingle into a natural dressing. A few drops of colatura di alici added to the oil is a sublime Campanian variation.
Adding balsamic vinegar (the cardinal sin). Using cold mozzarella straight from the fridge. Using tasteless out-of-season tomatoes. Shredding the basil. Adding dried herbs. Over-dressing with oil. Making it outside tomato season.
Il Cucchiaio d'Argento; Marcella Hazan, Essentials of Classic Italian Cooking