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.