Preheat oven to 400 degrees.
Press the thawed puff pastry sheet into an even square on a baking sheet that has been sprayed with cooking spray.
Use a paring knife to score a ½" border around the entire square of pastry.
In a small bowl, mix together the egg and water.
Use a pastry brush to lightly brush the egg wash around the scored "frame" of the pastry sheet.
Use the tines of a fork to dock holes inside the center of the puff pastry square.
Bake the prepared pastry sheet square in a preheated oven for 20 minutes (or until puffed and golden) *Note - the "frame" of the pastry sheet will be a deeper golden color due to the egg wash.
Remove the pastry sheet from the oven and use a spatula to press the center of the square lightly (while still hot) to highlight the "frame" around the pastry. Set aside.
In a medium bowl, mix together the mayonnaise, ranch dressing, Parmesan cheese.
Spread the mayonnaise mixture evenly inside the "frame" of the puff pastry square.
Top the mayonnaise mixture with the sliced tomatoes.
Season the tomatoes lightly with salt and pepper.
Evenly top the tomatoes with the drained and crumbled bacon and top the bacon with the assorted lettuce leaves.
Evenly drizzle the balsamic vinegar over the top of the tart and refrigerate until ready to serve.
*Note - This tart should only be made up to 4 hours in advance.
Cut into squares and serve as an appetizer or light dinner.