Preheat the Indoor/Outdoor Grill on Setting 4.
Brush one side of the flat bread with oil and place on one side of the preheated grill. On the opposite side, place the nectarine slices on the preheated grill. Place the lid on the grill. Grill the flatbread for 4 minutes on one side, and grill the nectarines for 2 minutes per side.
Brush oil on the top of the flatbread and flip over. Spread the apricot preserves on the flatbread and drizzle the mustard over the top. Place the nectarines on top of the preserves and mustard. Top with cheese and prosciutto.
Place the lid on the grill. Grill for 4-5 minutes, until cheese is melted.
Slice and serve immediately.