A Caprese salad is usually sliced tomatoes alternated with sliced mozzarella and basil leaves with olive oil and balsamic vinegar drizzled on top. It is really more of an antipasto than a traditional salad. This version takes it back into the salad world and it becomes a great dish to serve at any summer event, from a barbecue to a formal dinner.
The key to a great Caprese is great ingredients; since there are only a few, you need to make sure they really shine. Tomatoes have a wide range of quality going from tasteless to mind-blowing. Though recent advances in winter greenhouse tomatoes have dramatically increased the quality of off-season tomatoes, there is nothing like an in-season ripe local tomato.
Heirloom tomatoes tend to bruise more easily than regular tomatoes, but they have better flavor. The best way to tell is with your nose. This is true for basil and most other fruits and vegetables as well: how it smells is how it will taste.
Combine the vinegar, mustard, and garlic in a bowl and whisk well. While still whisking, add the oil in a thin stream. (This can also be done in a blender.) Season with salt and pepper, to taste.
In a large salad bowl, mix together the tomatoes, onion, mozzarella, and basil. Drain the liquid released by the tomatoes and toss with ¼-½ cup of the vinaigrette. Season with salt and pepper, to taste. Serve with crusty bread and basil pesto, if desired.