Preheat Oven: Preheat your oven to 400°F (200°C). Line a baking sheet with parchment paper.
Caramelize the Onions: In a large skillet, heat olive oil and butter over medium heat. Add sliced onions, sugar, salt, and pepper. Cook slowly, stirring occasionally, for 20–25 minutes until the onions are golden brown and sweetly caramelized. Stir in thyme at the end. Remove from heat and let cool slightly.
Prepare the Pastry: Roll out the puff pastry onto the prepared baking sheet. Score a 1-inch border around the edges without cutting all the way through.
Assemble the Tart: Spread the caramelized onions evenly within the scored border. Sprinkle with grated cheese, if using. Brush the edges of the pastry with beaten egg for a golden finish.
Bake: Bake in the preheated oven for 20–25 minutes or until the pastry is puffed and golden brown.
Serve: Allow the tart to cool for a few minutes before slicing. Serve warm or at room temperature.
Optional Additions and Customizations
Herb Variation: Add fresh rosemary, sage, or basil to the onions while cooking for a different aroma.
Cheese Twist: Swap Gruyère with goat cheese, feta, or parmesan for varied flavors.
Savory Extras: Top with caramelized mushrooms, roasted cherry tomatoes, or crispy bacon for extra richness.
Serving and Storage Tips
Serve as a starter, alongside soup or salad, or as part of a brunch spread.
Store leftovers in an airtight container in the fridge for up to 2 days. Reheat in the oven to retain crispness.
This Caramelized Onion Tart is a perfect balance of sweet, savory, and buttery goodness—a must-try for onion lovers and pastry enthusiasts alike.
