Preheat oven to 400°F.

Heat 1 tbsp of the olive oil in a small skillet on medium heat. Add onion slices and stir to coat with oil. Spread them out evenly over the pan and cook, stirring every few minutes, for 15-20 minutes until caramelized. You may have to reduce the heat to medium low during the process to prevent them from burning, depending on your stove. Remove from skillet and set aside.

In same skillet, heat the other tbsp of olive oil and add in mushrooms are garlic. Saute for 3 minutes, until just softened and fragrant. Remove from heat and set aside.

On a floured surface roll the two puff pastry together into a rectangle shape, mine ended up being approximately 13"x9". Place onto baking sheet, and with a knife lightly score the dough 1 inch from the edges, making a rectangle. On the inner rectangle, pierce with a fork. Bake for 10 minutes, then remove from oven.

While puff pastry is baking, bring a pot of water to a boil. Add in fiddleheads and boil for 5 minutes. Drain, and run cold water over the fiddleheads to cool them. Set aside.

Once you have removed the puff pastry from the oven, press down on center of dough to flatten if it is risen too much. Spread caramelized onions evenly over the dough, and then top with the cheese. Spread the mushrooms and fiddleheads even over top of cheese.

Bake for 20 minutes or until the crusts are golden.