Heat your oven to 350ºF. arrange the fillo shells on a baking sheet. Bake 10 minutes, or until crisped. Let cool.
In a small bowl, combine the cream cheese, vinegar, and parsley. Season with salt and pepper. Stir until smooth.
Melt the butter in a skillet set over medium heat. When the butter starts to foam, add the onion. Cook, stirring frequently, until the onion is very soft and golden brown, about 20 minutes. Remove from heat and let cool.
Fill the fillo shells with the cream cheese mixture. Top with caramelized onions. Serve at room temperature.