Save I stumbled onto this tart one October evening when I had nothing but onions, pastry, and time. The kitchen smelled like burnt sugar and butter by the time those onions turned amber, and I wondered if I'd ruined them. I hadn't. That first slice, warm and flaky, taught me that patience tastes better than perfection.
I made this for my neighbor after she helped me move a couch, and she stood in my doorway eating two slices before she even said hello. She told me it reminded her of something her grandmother used to make in Provence, which felt like the best compliment I could get. It became my go-to whenever I wanted to say thank you without spending all day in the kitchen.
Ingredients
- Puff pastry: Frozen puff pastry is your secret weapon here, thaw it in the fridge overnight so it rolls out without cracking.
- Yellow onions: They caramelize beautifully and turn golden without bitterness, slice them as thin as you can manage for even cooking.
- Butter and olive oil: The butter adds richness while the oil keeps it from burning during that long, slow cook.
- Sugar: Just a teaspoon coaxes out the onions natural sweetness and speeds up the browning.
- Crème fraîche: It gives a tangy creaminess that sour cream can mimic if that is what you have on hand.
- Dijon mustard: A tiny spoonful cuts through the richness and wakes up the whole tart.
- Gruyère cheese: It melts into nutty, savory pockets, Emmental or sharp cheddar work too if you want to switch it up.
- Fresh thyme: The earthy, floral notes tie everything together, dried thyme is fine but use half as much.
Instructions
- Preheat and prep:
- Set your oven to 400°F and line a baking sheet with parchment so nothing sticks. This is the moment to clear your counter and get organized.
- Caramelize the onions:
- Melt butter and oil in a large skillet over medium heat, then add the sliced onions, sugar, salt, and pepper. Stir often and let them cook slowly for 25 to 30 minutes until they turn a deep amber color and smell like candy.
- Roll the pastry:
- On a floured surface, roll your thawed puff pastry into a 12 inch square or circle and transfer it to the baking sheet. The edges can be rough, rustic is the whole point.
- Spread the base:
- Mix crème fraîche and Dijon mustard in a small bowl, then spread it evenly over the pastry, leaving a one inch border all around.
- Layer the filling:
- Scatter half the Gruyère over the crème mixture, pile on the caramelized onions, sprinkle with thyme, then top with the remaining cheese. Fold the pastry edges over the filling to make a casual border.
- Bake until golden:
- Slide the tart into the oven and bake for 20 to 25 minutes until the pastry puffs up and turns crispy gold. Let it cool for a few minutes before slicing so the filling settles.
Save 
Save The first time I served this at a dinner party, someone asked if I'd trained in France. I laughed and said no, just a lot of trial and error in a tiny kitchen. But that moment made me realize that good food does not need to be complicated, it just needs to be made with care.
Serving Suggestions
This tart shines alongside a simple green salad dressed with lemon and olive oil, or with a bowl of soup on a chilly evening. I have also cut it into small squares and served it as an appetizer with sparkling wine, and it disappeared faster than I could refill the glasses. It works hot, warm, or even at room temperature, which makes it ideal for potlucks or picnics where you can not control the timeline.
Storage and Make Ahead
You can caramelize the onions a day or two ahead and keep them in the fridge, which makes assembly quick when you are ready to bake. The baked tart holds up well for a day wrapped in foil, just reheat it in a low oven to crisp up the pastry again. I would not freeze the finished tart because the pastry gets soggy, but you can freeze the caramelized onions on their own and thaw them when inspiration strikes.
Variations and Swaps
If you want to add a splash of balsamic vinegar to the onions in the last five minutes of cooking, it gives them a sweet-tart edge that is really lovely. You can swap the Gruyère for any good melting cheese you have, and I have even used feta crumbled on top for a sharper, saltier bite. Some people like to add crispy bacon or prosciutto for a non-vegetarian twist, and honestly, it is hard to argue with that.
- Stir in a handful of fresh spinach or arugula before baking for a bit of green.
- Drizzle honey over the finished tart if you like a sweet-savory contrast.
- Use red onions instead of yellow for a deeper color and slightly different flavor.
Save 
Save This tart has become my answer to so many occasions, from lazy Sundays to last minute guests. I hope it finds a place in your kitchen the way it did in mine.
Recipe FAQ Section
- → How do you caramelize onions properly?
Slowly cook thinly sliced onions over medium heat with butter and olive oil, stirring often until they turn deep golden and soft, usually about 25–30 minutes.
- → Can I use a different cheese than Gruyère?
Yes, Emmental or sharp cheddar can be good substitutes, providing slightly different flavors but maintaining a creamy melt.
- → What is the purpose of Dijon mustard in the tart?
Dijon mustard adds a subtle tang and depth to the creamy base, enhancing the overall flavor without overpowering the onions.
- → How to ensure a crispy pastry crust?
Chilling the puff pastry before baking, using parchment paper, and baking until golden ensures a flaky and crisp crust.
- → Can this tart be served at room temperature?
Yes, the tart tastes delicious warm or at room temperature, making it versatile for different occasions.