This is my favorite sort of recipe. It’s simple, easy to adapt to what you have on hand, and can be assembled ahead of time. It's perfect for an afternoon snack or party with friends and family.
If your puff pastry sheets are frozen, thaw them according to the package’s directions. Cold puff pastry is much easier to work with, so keep the thawed puff pastry chilled in the refrigerator until you are ready to use.
This onion tart is assembled and then baked upside down. By baking it upside down, the onions will be in contact with the hot baking sheet. They will caramelize and the toppings will embed themselves into the puff pastry while it puffs up light and flaky on top.
Unroll your chilled puff pastry, taking care not to stretch the pastry sheet as you unroll it. Most brands are rolled around a piece of parchment paper. We’ll use that paper to hold the pastry while the toppings are added to it.
Drizzle the olive oil on top of the pastry. You can spread the olive oil to cover the surface of the pastry or leave it as is. It will naturally spread as it bakes. Separate your sliced onion into individual pieces. Place the onion in a single layer on top of the pastry. You want to cover as much of the pastry as possible in order to add the delicious onion flavor to each bite while keeping the onions in a single layer so that they bake evenly.
Add the herbs, cheese, and small dollops of marinara sauce, distributing them evenly over the onions. Drizzle with balsamic vinegar or glaze and season with salt and pepper. This is your moment to make this recipe your own and add other flavorings that you love. As long as they are kept in a single layer, they will bake up beautifully with the onions.
At this point, you can bake the pastry immediately or place it in the refrigerator to bake later. If you are making it ahead of time, it can be held for several hours in the refrigerator. I like to prepare them ahead of time for the holidays and keep several half sheets stacked and separated by parchment paper on a tray in the refrigerator until I am ready to bake them.
When you are ready to bake, preheat the oven to 350 degrees Fahrenheit. Position your oven racks in the top and bottom third of the oven if you are baking multiple sheets or in the center if you have a single sheet to bake.
Place a sheet of parchment paper on top of the assembled onion tart. Place your baking sheet on top of the parchment and quickly flip the puff pastry so that the toppings are on the bottom. Remove the paper backing from the puff pastry.
Using a sharp knife or pizza cutter, slice the puff pastry into individual portions. You can make them as large or small as you like. Apply enough pressure to cut through the onions without cutting through the parchment paper below. There’s no need to separate the portions. They will shrink slightly as the bake, creating a natural separation from each other.
In a small bowl, whisk the egg to make an egg wash. You can add a pinch of salt if you like to help make it easier to break the egg apart. Using a pastry brush, brush the top of the puff pastry, covering it completely with a light coating of the egg wash. While the egg wash is wet, you can add a bit of salt, pepper, herbs, and Parmesan if you like.
Transfer your baking sheet to the preheated oven. Bake for 16 to 18 minutes until the puff pastry is light, flaky, and golden brown. To ensure even browning, rotate the baking sheet halfway through baking.
Remove the baking sheet from the oven and allow the tarts to cool undisturbed for five minutes. Serve and enjoy.
Notes
THE HERBS I like to use a combination of fresh rosemary, thyme, and basil for these tarts because I always have them on hand. You can use the fresh herbs you have growing in the garden or substitute a teaspoon of dried herbs if you prefer.ADAPTING THE RECIPE Other than the puff pastry, this recipe is adaptable from start to finish. You can add more of the ingredients you love and omit those that you don’t. You can use your favorite herbs, cheeses, and toppings to create your own delicious version. As long as the ingredients are in a single layer under the puff pastry, you can add as much of them as you like. There's no need for precise measurements to make this recipe.During the summer when tomatoes are ripe in the garden, I like to add sliced cherry tomatoes instead of the marinara sauce. You can use what you like and what you have on hand to make this recipe your own.SCALING THE RECIPE You can bake multiple sheets at the same time for a party or holiday gathering just as easily as you can make a half or quarter sheet for two and save the remaining pastry for another day or another use. The ingredients listed make one full size sheet of puff pastry but can easily be adjusted to make multiple sheets or a part of a sheet.