Preheat the oven to 375 degrees F.
Place the puff pastry on a flour-dusted surface and roll until big enough to wrap around the bar of cream cheese. Spread the ranch dressing over the outside of the cream cheese, and then coat with the chopped bacon. Place in the center of the puff pastry. Sprinkle the green onion if using over the top of the cream cheese. Fold the pastry up to cover the cream cheese, sealing the seam with your fingertips and a few drops of water. Tuck the hanging edges under the cream cheese to make a smooth rectangular shape. Brush with beaten egg or milk if you want a shiny finish.
Place the bar in a small oven-safe serving dish and bake until the pastry is golden and the cream cheese is soft, 30 to 35 minutes. Serve with crostini and vegetables.