In a frying pan or sauté pan, stir together the sliced onions, dried cranberries, 3 tablespoons balsamic vinegar, water and brown sugar. Bring to a boil, then cover pan and lower heat to a simmer. Cook the onions for about 10 to 15 minutes, stirring periodically, until the onions are caramelized.
Whisk together the remaining tablespoon of balsamic vinegar and agave nectar/honey. Set aside.
Heat the flatbread on the grill, in the oven, or on a stovetop grill pan. Flip it over and top with the caramelized onions, and sprinkle with feta cheese. Arrange the Serrano ham or prosciutto on top. Grill/Bake/Cook the flatbread (cover if using grill or stovetop grill pan) for about 5 minutes, or until the cheese has started to melt.
Transfer to a board, top with a handful of arugula, and drizzle with balsamic glaze. Cut into slices and serve immediately.