To a medium skillet, heat two tablespoons of olive oil over medium-low heat. Add the onions and cook for 15 minutes, stirring frequently. Add a pinch of salt and cook for 30 minutes more, stirring frequently, and adding more olive oil as needed, or until the onions are a golden brown color. Transfer the caramelized onions to a small bowl and set aside.
In a medium bowl, combine the albacore tuna, mayonnaise, mustard, diced apple, black pepper and juice from the lemon. Mix and salt and pepper to taste.
On a foil-lined baking sheet, lay out the slices of bread. Divide the caramelized onions among the slices of bread. Top with a few scoops of tuna; and then a liberal handful of provolone. Turn on the broiler and when hot carefully place the baking sheet under the broiler and cook until the cheese is melty and lightly browned. Top with a few potato chips and enjoy.