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Flavor-Packed Caramelized Onion Tart

Helen T. Patterson
A buttery tart crust layered with sweet caramelized onions, melted Gruyère, and a creamy egg custard makes this tart a show-stopping appetizer.
Quick to prepare and packed with protein, fiber, and heart-healthy fats, it’s perfect for entertaining, weekday meals, or meal prep. Serve warm or at room temperature for maximum flavor.
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Prep Time 40 minutes
Cook Time 30 minutes
Total Time 1 hour 10 minutes
Course Appetizer
Cuisine American, French
Servings 8

Equipment

  • 1 10-inch tart pan with removable bottom
  • Large sauté pan
  • Parchment paper
  • Pie weights (or dried beans)
  • Small mixing bowl
  • Whisk
  • Spatula

Ingredients
  

  • 2 large sweet onions halved and thinly sliced
  • 1 tablespoon olive oil
  • 4 sprigs fresh thyme divided
  • ½ teaspoon kosher salt
  • ⅛ teaspoon ground black pepper
  • 1 premade pie crust
  • 2 large eggs
  • 3 tablespoons cream
  • 5 ounces Gruyère cheese shredded (about 1½ cups)

Instructions
 

  • Prepare the Tart Pan: Begin by preheating your oven to 375°F (190°C) and positioning the oven rack in the middle.
    Lightly grease a 10-inch tart pan with a removable bottom using cooking spray or a thin layer of butter.
    This ensures the tart will release easily once baked. Set the prepared pan aside while you work on the filling.
  • Slice the Onions: Take your sweet onions and cut them in half from top to root.
    Peel away the outer skin and then slice each half into thin, uniform half-moons.
    Uniform slices help the onions cook evenly and caramelize without burning.
  • Sauté the Onions: In a large sauté pan over medium-high heat, add 1 tablespoon of olive oil.
    Once the oil is hot but not smoking, add the sliced onions along with 2 sprigs of fresh thyme, ½ teaspoon kosher salt, and ⅛ teaspoon black pepper.
    Stir to coat the onions evenly with oil and seasonings. Sauté the onions for about 5 minutes, allowing them to begin softening and releasing their natural sweetness.
  • Wilt the Onions with Water: After the initial sauté, pour in ¼ cup of water and cover the pan with a lid.
    Let the onions steam for 10 minutes.
    This gentle cooking method helps break down the onions without browning too quickly, creating a tender base for caramelization.
  • Caramelize the Onions: Remove the lid and reduce the heat to medium.
    Continue cooking the onions, stirring occasionally, until they are deeply soft, richly golden, and fully caramelized, which usually takes about 20 more minutes.
    Keep an eye on them to prevent burning, and scrape the bottom of the pan as needed. Once finished, remove the thyme sprigs from the pan.
  • Blind-Bake the Tart Crust: While the onions are cooking, press the premade pie crust into the tart pan, ensuring it reaches halfway up the sides.
    Trim any excess dough. Line the bottom of the crust with parchment paper and fill it with pie weights or dried beans to prevent bubbling during baking.
    Place the crust in the oven and bake for 15–20 minutes, until it starts to turn a light golden color.
    Remove from the oven and let it cool slightly before removing the parchment and weights.
  • Reduce Oven Temperature: Once the crust is partially baked, lower the oven temperature to 350°F (175°C) to prepare for the filling.
    This lower heat allows the egg custard to set gently without overcooking the onions or browning the tart too quickly.
  • Prepare the Cheese and Herbs: Shred 5 ounces of Gruyère cheese (about 1½ cups).
    Remove the leaves from the remaining 2 sprigs of thyme and mix them into the shredded cheese.
    This simple step infuses the cheese with subtle herbal notes, enhancing the overall flavor of the tart.
  • Make the Egg Custard: In a small mixing bowl or measuring cup, whisk together 2 large eggs and 3 tablespoons of cream until smooth and fully combined.
    This mixture will create a silky custard that binds the onions and cheese together while adding a creamy texture to the tart.
  • Assemble the Tart: Spread the cheese and thyme mixture evenly across the bottom of the pre-baked tart crust.
    Next, carefully layer the caramelized onions over the cheese, ensuring they cover the entire surface evenly. Finally, pour the egg and cream mixture gently over the onions.
    Use a spatula to spread it evenly, making sure the custard seeps into all gaps without overflowing the crust.
  • Bake the Tart: Place the assembled tart back in the oven and bake at 350°F (175°C) for about 30 minutes.
    The tart is done when the custard is fully set, the top is lightly golden, and the edges of the crust are crisp and slightly browned.
  • Cool and Serve: Remove the tart from the oven and allow it to cool for at least 10 minutes before slicing.
    You can serve it warm for a soft, creamy texture or at room temperature for a slightly firmer bite.
    Garnish with additional thyme if desired, and enjoy this savory, elegant appetizer with friends and family.

Notes

  • Best enjoyed the same day for peak flavor and texture.
  • Use a sharp knife when slicing to keep the crust intact.
  • Caramelized onions can be prepared a day ahead and stored in an airtight container to deepen the flavor.
  • Allow the tart to cool slightly before removing from the pan for a crisper crust.