2largeVidalia onionsor other sweet onion, thinly sliced
Preheat oven to 400 degrees and line a baking sheet with parchment paper. Set aside.
In a saute pan over medium heat, add olive oil. When hot, add sliced onions. Cook for 10 minutes, stirring occasionally.
Reduce heat to medium-low and add beef broth, salt, and pepper. Stir to incorporate. Cook down onions for an additional 25 minutes, stirring occasionally. If pan begins to get dry, add water 1 tablespoon at a time and lower heat slightly, scraping onions off the bottom of the pan with a spatula. Onions are done when they are a rich brown color. (You can cook down your onions up to an hour if you desire. Continue adding water, stirring and scraping your onions as needed).
Unfold thawed puff pastry. Using a rolling pin, roll a few times each way to remove creases. Lay puff pastry onto prepared baking sheet.
Pour onions over prepared puff pastry and spread into an even layer, leaving a 1 inch crust around the edge. Sprinkle gorgonzola cheese and fresh thyme evenly over onion layer.
Bake for 22-25 minutes, or until crust and cheese are lightly browned and puff pastry is cooked through in the center.
Remove from oven, slice, and serve. Tart can be served warm or at room temperature.
Optionally, top your onion tart with fresh arugula or chopped parsley before serving.
Gorgonzola substitutions: any blue cheese, feta (if you're opposed to the strong flavor of blue cheese), gruyere, or goat cheese.
Caramelized onions can be prepared the day beforehand and stored in a sealed container in the refrigerator until ready to assemble tart.