Take off the heat and leave to cool. You can then blend it for a smooth consistency, or keep it as is for a rustic chunky style. I left mine chunky.
Brush your bread slices with some Belmorso EVOO, and grill each one. You can use a grilling pan, or the same pan you used for your onions. It will infuse the bread with that flavour.
Top each slice with the balsamic onion jam you made.
Add one or two slices of prosciutto crudo on top of the onions, depending on how large each is. Mine were small, so I added two.
I used a potato peeler to shave some pecorino cheese on top of the prosciutto. You can use a proper cheese shaver.