Preheat the broiler.

Clean the leeks thoroughly, and boil in salted water 6 to 10 minutes, until cooked but still firm, then drain.

Put the leeks in a baking dish, and cover with a layer of basil leaves. Cut the mozzarella into 1/4-inch slices, and place atop the basil layer. Put the dish under the preheated broiler, and watch carefully. In 3 to 5 minutes the cheese should start to melt and brown; at this point, remove the dish.

Mix the oil and vinegar and drizzle over the mozzarella. Season with salt and pepper to taste. Serve immediately with a slice of country bread.