Pour a cup of balsamic vinegar in a small saucepan over medium heat. Bring the heat to a boil without reducing the heat on the stove.
At this point, turn down the heat so that boil reduces to a simmer. Stir occasionally and allow to simmer until the vinegar has reduced by at least half (for a thinner reduction) or more (for a more syrupy consistency). Allow to cool and transfer to an airtight container; store in the refrigerator until ready to use.
Heat your broiler in the oven.
Lightly brush both sides of the bread slices with olive oil. Grill the bread until golden brown, about 2 minutes, then turn and grill the second side about 1 to 2 minutes.
Mix the cheeses together and season with salt and pepper. Divide this mixture and spread over the 4 slices of bread. warm for 2 minutes in the oven.
Top each with three slices tomatoes. Divide the arugula onto the tomatoes slices. Sprinkle with salt and cracked black pepper. Drizzle over a little balsamic vinegar reduction. Top each sandwich with two slices of Iberico, Serrano or prosciutto.