Awesomeness incarnate
This is a simple and traditional italian classic. It's guaranteed to wow at any bbq, and will take you a half of a second to prepare if you're fast with a knife. This is somewhat a hallmark of italian cooking. Sweet and flavorful, the simplicity of the dish lets the ingredients shine. The aesthetics of the dish are designed to match the colors of the italian flag.
Ingredients:
Sliced tomatoes (for most of these style of dishes I recommend campari on the vine tomatoes. Vine ripened tomatoes are sweeter, fresher, and I find the taste to be incomparable. Traditionally, insalata caprese is made with plum tomatoes. I find the campari tomatoes to be a better match. Slice them a bit thick.)
sliced mozarella (really, the higher quality the better. round is more aesthetically pleasing, and balls of mozarella tend to be higher quality)
fresh basil (chop it if you wish, I find whole leaves to be more aesthetically appealing. Get the small leaf variety. The big hydroponic leaves are less flavorful, and are better used in cooked dishes)
extra virgin olive oil
ground pepper to taste (some people use salt, I find the mozarella provides all the salt you need. Traditionally salt is added, so use your own judgement. I say try it my way first, then adjust the salt to your pallate)
This is more of an arrangement than a cooking process. Slice the tomatoes and mozarella to about the same thickness, and place them on a plate. you want to follow the color theme of the flag of italy. So, place a tomato slice, place some mozarella, then place a basil leaf. Repeat in a ring around the edge of the plate, then put another, smaller circle of this arrangement in the center. Garnish the very middle of the dish with a few leaves of basil, still attatched together. I usually use a sprig of 3 small leaves.
Drizzle the whole thing with a light sprinkle of extra virgin olive oil, and grind some fresh pepper onto it. You don't want this to sit in the oil for an amazing amount of time, so I'd suggest if you're bringing it to a bbq, take a bit of olive oil with you in an empty water bottle or something and cover the hole with your thumb, leaving a tiny opening for you to shake the oil out of.
I suggest against vinegar, or a vinaigrette, for this recipe. It might taste ok, and isn't awful, but you're burying all of the subtle flavors and complexities of the basic dish.