In a nonstick skillet on medium low, heat butter until just melted, add onions and a pinch of salt. Gently stir and let onions cook until translucent. Stir in brown sugar, balsamic vinegar and reduce heat to low. Let cook down until caramelized, approximately 20 minutes. Remove from heat and let cool. Meanwhile, to assemble the tart, begin by heating the oven to 375 degrees. Place the half sheet of puff pastry on a parchment paper lined baking tray. Dock the pastry with a fork and spoon the cooled caramelized onions on top. Take the sliced potato and place on top of the pastry and onions, slightly overlapping each slice. Next, place bacon on top of the potato and dot half of the cream cheese on top of the bacon. Season with salt and pepper and bake in the oven for 30 - 40 minutes or until the pastry and bacon are golden brown. Remove from oven, place on serving platter or cutting board and dot the remaining cream cheese on top followed by the parsley. Slice and serve immediately.