Note: Saba, an unfermented syrup made from grape must, has a non-cloying, caramelized sweetness, which is preferable to the more acidic balsamic vinegar; however, you could try a thick, sweet balsamic or balsamic reduction if you don't have saba.
4 (4-inch-long) slices good bread
1 (4-ounce) ball fresh mozzarella
1 large nectarine
4 large or 8 medium leaves basil (plus more for garnish, if desired)
Fleur de sel
Brush both sides of the bread with olive oil and grill or broil until browned.
Cut the mozzarella ball in half and cut each half into 6 slices. Halve and pit the nectarine and cut each half into 6 slices.
Assemble each tartine in this order: bread, 3 slices mozzarella, 1 or 2 basil leaves, 3 slices nectarine. Then drizzle saba on top and lightly sprinkle with salt (not too much – just a hint). Serve immediately.
Related: Peaches and Goat Cheese
(Images: Emily Ho)