Combine onion, balsamic and basil in a bowl and set aside for 10 minutes.
Meanwhile, drizzle bread with 1½ tbs oil. Cook on a chargrill, barbecue or in a frying pan, tossing frequently until golden. Place into a large bowl.
Combine cucumber, peach wedges and salad mix in the bowl with bread. Add onion, reserving the balsamic dressing. Stir remaining oil into the balsamic and season with salt and pepper.
Add prosciutto and dressing to salad. Toss gently to coat and serve.
