Options for hot dog toppings are literally infinite, and this topping is a play on the traditional New York-style onion sauce. The onions are cooked down, tomato paste, mustard, and balsamic vinegar are added, and the tangy results perfectly complement a plump broiled hot dog.
Heat the butter in a sauté pan over medium heat and add the onions and pinch of salt until softened. Add the tomato paste, and cook for about 2 minutes. Add the mustard, balsamic vinegar, and water. Season with pepper, to taste, stir until combined, and simmer until it becomes a thick sauce. Season with salt, to taste.
Add the franks to the buns, and top with a small amount of mustard, if desired. Spoon the onion sauce on top.