Prepare the balsamic glaze: Pour the balsamic vinegar into a small saucepan. If using honey or maple syrup, whisk it in now. Bring the vinegar to a simmer over medium heat. Reduce heat to low and let it gently bubble, stirring occasionally, until it thickens enough to coat the back of a spoon (about 8-12 minutes). It should reduce by about half. Remove from heat and set aside to cool; it will thicken further as it cools.

Slice the nectarines: Wash and dry the nectarines. Cut them into uniform slices, about 1/4 inch thick.

Assemble the salad components: On a large serving platter, arrange the nectarine slices, alternating with slices or small pieces of fresh mozzarella. Aim for an appealing, visually balanced pattern.

Season and garnish: Tuck the whole fresh basil leaves among the nectarines and mozzarella. Sprinkle evenly with sea salt and freshly ground black pepper.

Finish and serve: Once the balsamic glaze has cooled completely and reached a syrupy consistency, drizzle it generously over the entire salad. Serve immediately for the best flavor and texture.