Save The first time I made this tart, I was rushing to throw something together for unexpected guests. I had onions, a sheet of puff pastry in the freezer, and not much else. That night taught me that simple ingredients, treated with patience and care, can become something extraordinary.
My friend Marie stayed late in the kitchen that evening, hovering near the stove as the onions slowly transformed. She kept asking if they were done yet, not understanding that the magic happens in those last ten minutes when the sugars deepen and the fragrance fills the entire room. We ended up eating the tart standing at the counter, warm slices in hand, talking until midnight.
Ingredients
- Frozen puff pastry (1 sheet, about 250 g): Keep it frozen until you need it, then thaw in the refrigerator for best results. Cold pastry handles better and bakes up flakier.
- Large yellow onions (3), thinly sliced: Yellow onions sweeten beautifully as they cook. Take your time slicing them evenly so they caramelize at the same rate.
- Unsalted butter (2 tbsp) and olive oil (1 tbsp): The combination prevents the butter from burning while adding rich flavor. Use both.
- Sugar (1 tsp), salt (1/2 tsp), and pepper (1/4 tsp): Just a touch of sugar helps the onions brown. Season as you go for layers of flavor.
- Crème fraîche or sour cream (1/2 cup): Crème fraîche has a tangy richness that balances the sweet onions. Sour cream works in a pinch.
- Dijon mustard (1 tsp): A small amount adds a sharp contrast that cuts through the richness of the pastry and cheese.
- Grated Gruyère cheese (1/2 cup): Gruyère melts beautifully with a nutty flavor. Emmental or sharp cheddar are good substitutes.
- Fresh thyme leaves (1 tsp): Thyme pairs perfectly with onions and cheese. Fresh herbs make a noticeable difference here.
- Fresh chives (1 tbsp, optional): A finishing touch of color and mild onion flavor. Add them right before serving.
Instructions
- Heat the oven and prepare the pan:
- Preheat to 400°F (200°C). Line a baking sheet with parchment paper so nothing sticks and cleanup is effortless.
- Caramelize the onions slowly:
- Melt butter with olive oil in a large skillet over medium heat. Add sliced onions with sugar, salt, and pepper. Cook, stirring often, until deeply golden and fragrant, about 25 to 30 minutes.
- Roll out the pastry:
- On a lightly floured surface, roll the puff pastry into a 12-inch square or circle. Transfer to the prepared baking sheet.
- Spread the base layer:
- Mix crème fraîche and Dijon mustard in a small bowl. Spread evenly over the pastry, leaving a 1-inch border for the edges.
- Layer the toppings:
- Scatter half the Gruyère over the crème mixture. Arrange caramelized onions on top and sprinkle with thyme. Finish with remaining cheese.
- Create the rustic border:
- Fold the pastry edges gently over the filling, pleating slightly where needed. The center stays open, the edges form a casual crust.
- Bake until golden:
- Bake for 20 to 25 minutes until the pastry is puffed, golden, and crisp. Cool for at least 5 minutes before slicing.
- Finish and serve:
- Garnish with fresh chives if using. Serve warm or at room temperature. The flavors develop as it rests.
Save This tart has become my go-to when I want something that feels special but doesnt require hours of attention. I have made it for Sunday brunch, weeknight dinners, and last minute guests. Every single time, someone asks for the recipe.
Making It Ahead
You can caramelize the onions up to two days in advance and store them in the refrigerator. The flavors actually improve after sitting overnight. When you are ready to bake, just assemble and bake as directed.
Serving Suggestions
This tart works beautifully as a light lunch with a simple green salad dressed in vinaigrette. For appetizers, cut it into smaller squares and serve slightly warm. A crisp white wine like Sauvignon Blanc cuts through the richness perfectly.
Common Questions
Can I use store bought dough? Yes, puff pastry from the freezer section works wonderfully. Just let it thaw in the fridge, not on the counter, so it stays cold and easy to handle. What if I do not have crème fraîche? Sour cream is a fine substitute. The texture will be slightly thinner but the flavor will still work.
- Do not skip the mustard, even if you do not usually like it. It almost disappears into the background but makes the other flavors pop.
- Let the tart cool for at least five minutes before slicing so the filling sets and the pastry stays crisp.
- Leftovers reheat well in a 350°F oven for about 10 minutes.
Save There is something deeply satisfying about transforming ordinary onions into something worthy of a dinner party. This tart proves that patience and a few good ingredients are often all you need.