Save I stumbled on this tart one autumn evening when I had three onions turning soft in the pantry and a forgotten sheet of puff pastry in the freezer. The kitchen filled with that sweet, deep smell of caramelizing onions, and I knew I was onto something good. My neighbor wandered over halfway through baking, drawn by the scent drifting through the window. We ended up sharing slices warm from the oven with cold white wine, and she still asks me to make it every time she visits.
The first time I made this for a dinner party, I was running late and almost panicked. But I learned that this tart is forgiving, the pastry crisps up beautifully even if you fold the edges unevenly, and the onions taste even better if you start them early and let them sit while you prep everything else. One guest told me it reminded her of a bistro in Lyon, and I didnt have the heart to tell her Id never been.
Ingredients
- Frozen puff pastry: Thaw it in the fridge overnight or on the counter for about 30 minutes, it should unfold easily without cracking.
- Yellow onions: They have the perfect balance of sweetness and sharpness for caramelizing, slice them thin so they cook evenly.
- Butter and olive oil: Using both gives you rich flavor and prevents the butter from burning during the long cook.
- Sugar: Just a pinch helps the onions caramelize faster and deepens their golden color.
- Creme fraiche: It adds tang and creaminess, but sour cream works just as well if thats what you have.
- Dijon mustard: A small spoonful cuts through the richness and wakes up the whole tart.
- Gruyere cheese: It melts beautifully and has that nutty, slightly sweet flavor that loves onions.
- Fresh thyme: The leaves release their fragrance in the oven and tie everything together.
Instructions
- Get the oven ready:
- Preheat to 400F and line your baking sheet with parchment so the tart releases easily. This also makes cleanup almost effortless.
- Caramelize the onions:
- Melt butter and oil together in a large skillet over medium heat, then add the onions with sugar, salt, and pepper. Stir every few minutes, letting them soften and turn deep gold, this takes patience but the smell alone is worth it.
- Prepare the pastry base:
- Roll out your thawed puff pastry on a floured surface to about 12 inches. Lift it gently onto the parchment lined sheet.
- Spread the creamy layer:
- Whisk together creme fraiche and Dijon, then spread it over the pastry, leaving a border for folding. The mustard gives it a subtle kick.
- Build the tart:
- Sprinkle half the Gruyere over the cream, pile on the caramelized onions, scatter thyme, then top with the rest of the cheese. Fold the pastry edges up and over to frame the filling.
- Bake until golden:
- Slide it into the oven for 20 to 25 minutes until the pastry puffs and turns crisp and the cheese bubbles. Let it cool just a bit before slicing so the filling sets.
Save I remember cutting into this tart on a rainy Sunday afternoon, the pastry shattered under my knife, and the smell of thyme rose up with the steam. My daughter, who usually avoids onions, took a bite and asked for seconds. It became one of those recipes that feels like a small victory every time I pull it from the oven.
Storing and Reheating
Wrap any leftover slices in foil and keep them in the fridge for up to three days. Reheat in a 350F oven for about 10 minutes to crisp the pastry back up, microwaving makes it soggy. I have also eaten cold slices straight from the fridge for breakfast, and I wont apologize for it.
Variations to Try
Stir a splash of balsamic vinegar into the onions during the last few minutes for a hint of sweetness and tang. Swap Gruyere for sharp cheddar or Emmental if thats what you have on hand. You can also add crumbled bacon or thinly sliced mushrooms to the filling, both love the company of caramelized onions.
Serving Suggestions
This tart shines on its own, but I like to serve it with a handful of arugula dressed in lemon and olive oil. A glass of chilled Sauvignon Blanc or a light Pinot Grigio cuts through the richness beautifully.
- Slice it into small squares for an elegant appetizer at gatherings.
- Pair it with a simple green salad for a satisfying weeknight dinner.
- Serve it at room temperature on a picnic blanket with fresh fruit and cheese.
Save This tart has become my go to when I want something that feels special without the fuss. I hope it fills your kitchen with the same warmth it brings to mine.
Recipe FAQs
- → How do I achieve perfectly caramelized onions?
Cook sliced onions slowly over medium heat with butter and olive oil, stirring often. Adding a pinch of sugar enhances caramelization, typically taking 25-30 minutes to reach a deep golden color.
- → Can I substitute Gruyère cheese?
Yes, Emmental or sharp cheddar work well as alternatives, providing similar melting qualities and flavor profiles.
- → What is the best way to prepare the puff pastry?
Thaw puff pastry sheet fully before rolling to a 12-inch square or circle. Avoid overworking to keep it flaky and tender after baking.
- → How can I add extra flavor to the tart?
Stir in a splash of balsamic vinegar to the onions during the last few minutes of caramelizing for added depth and a slight tang.
- → What herbs complement this tart best?
Fresh thyme leaves are used in the filling, while finely chopped chives add a fresh garnish, enhancing the savory aroma and presentation.