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Sliced caramelized onion and Gruyere tart on a rustic wooden board, showing golden puffed pastry and melted cheese.

Caramelized Onion and Gruyere Tart: Rustic and Buttery

Puff pastry topped with slow-caramelized onions and grated Gruyere, baked until golden and bubbling.
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Prep Time 15 minutes
Cook Time 1 hour
Total Time 1 hour 20 minutes
Servings: 6 servings
Course: Appetizer
Cuisine: French
Calories: 70
Ingredients Method Notes

Ingredients
  

Caramelized Onions
  • 3 tbsp unsalted butter (42 g)
  • 4 yellow onions large (about 900 g); thinly sliced
  • 1/2 tsp salt
  • 1 tbsp balsamic vinegar
  • 1 tbsp fresh thyme leaves plus extra for garnish
Tart Assembly
  • 1 all-butter puff pastry sheet (about 250 g); thawed
  • 150 g Gruyere cheese grated
  • 1 egg large; beaten, for egg wash
  • 1/4 tsp black pepper freshly ground

Method
 

Caramelize the Onions
  1. Melt the butter in a 12-inch / 30 cm skillet over medium heat. Add the sliced onions and salt, stirring to coat them in the butter.
  2. Reduce the heat to medium-low and cook the onions for 35 to 40 minutes, stirring every 5 minutes, until they turn deep amber and jammy. If they start to catch on the bottom, lower the heat and add a splash of water.
  3. Stir in the balsamic vinegar and thyme, scraping up any browned bits from the bottom of the skillet. Cook for 2 more minutes, then remove from the heat and let cool for 5 minutes.
Assemble and Bake the Tart
  1. Heat the oven to 200 C / 400 F and line a sheet pan (33 x 46 cm / 13 x 18 inch) with parchment paper.
  2. Unroll the puff pastry onto the sheet pan. Using a knife tip, score a border about 3/4 inch / 2 cm from the edge, cutting only partway through the dough.
  3. Scatter half the Gruyere over the pastry inside the border. Spread the cooled caramelized onions evenly over the cheese, then top with the remaining Gruyere and the black pepper.
  4. Fold the scored border up slightly and brush it with the beaten egg.
  5. Bake for 20 to 22 minutes, until the pastry is puffed and deep golden brown and the cheese is bubbling at the edges.
  6. Let the tart cool on the pan for 5 minutes, scatter extra thyme over the top, then slice into 6 wedges and serve warm.

Notes

  • Cook onions low and slow; rushing the heat gives pale, sharp-tasting onions instead of sweet ones.
  • Keep puff pastry cold until it goes in the oven for the best rise.
  • Don't skip the balsamic, it cuts the richness of the butter and cheese.
  • Score, don't cut, the pastry border so it puffs into a raised edge.