069 / 100 · Vegetables Take the Lead

Caramelized Onion and Gruyere Tart

1 nine-inch tart, cut into 6 wedges Active 40 min Total 2 hr 30 min

Sweet onions and nutty Gruyere give this tart a generous flavor without a complicated filling. The onions cook slowly until they are soft, reduced, and golden at the edges, then settle into a simple egg custard. A homemade shortcrust holds a clean wedge and gives the soft interior something crisp to lean against. The dough rests while the onions cook, making this a useful leisurely lunch project rather than a last-minute dish. Let the baked tart stand before cutting; the custard firms slightly and the onion layers become easier to see.

Jump to the ingredients

A slice of caramelized onion and Gruyere tart beside the remaining golden tart.

Golden onions, not scorched onions

Three large onions look excessive at the start but lose much of their volume as water evaporates. Use a wide skillet and stir more often as they darken. If a sticky brown layer appears on the pan, add a tablespoon of water and scrape it back into the onions. That browned sweetness belongs in the tart. Blackened onion edges taste bitter and cannot be corrected by the cheese.

Give the pastry a head start

Blind baking means baking the empty shell before the filling goes in. The weights keep the base level, and the short second bake without weights dries its surface. Together these steps keep a wet custard from soaking raw dough. Set the tart pan on a baking sheet before filling it, particularly if the pan has a removable bottom. Serve wedges warm with a simple green salad, or let them cool for a picnic-style lunch kept properly chilled until serving.

Plan the order of work

Start the dough first, then slice the onions while it chills. Their slow cooking can happen during the pastry preparation, so the stages do not need to fill the whole afternoon. Mix the custard only when the shell is nearly ready. If the onions finish well ahead, spread them on a plate to cool more quickly; hot onions can begin setting the eggs before the filling is evenly distributed. A removable-base pan makes a handsome edge, but always support its bottom with your hand or a tray when moving it.

Ingredients

  • 1 1/2 cups all-purpose flour (180 g), plus a little for rolling
  • 1/2 teaspoon fine salt, divided
  • 1/2 cup cold unsalted butter, cubed (113 g)
  • 3–4 tablespoons ice water
  • 1 tablespoon olive oil
  • 3 large yellow onions, thinly sliced (about 750 g prepared)
  • 1 teaspoon fresh thyme leaves
  • 3 large eggs
  • 1/2 cup heavy cream (120 ml)
  • 1/2 cup whole milk (120 ml)
  • 1 1/2 cups shredded Gruyere cheese (150 g)

Set out

9-inch fluted tart pan with removable base, about 1 1/4 inches deep, Rolling pin, Large skillet, Baking sheet, Parchment and pie weights or dried beans, Mixing bowl and whisk

Allergens

Wheat, milk, egg. Check ingredient labels and substitutions.

Make it

  1. Mix the flour and 1/4 teaspoon salt in a bowl. Rub in the cold butter until the mixture resembles coarse crumbs with a few pea-size pieces. Add 3 tablespoons ice water and mix; add the remaining tablespoon only if needed to bring the dough together. Flatten into a disk, wrap, and chill for 30 minutes.
  2. Meanwhile, warm the oil in a large skillet over medium-low heat. Add the onions and remaining 1/4 teaspoon salt. Cook for 30–35 minutes, stirring often, until soft and golden. Add a tablespoon of water whenever the pan catches. Stir in the thyme, remove from the heat, and cool for 10 minutes.
  3. Heat the oven to 375°F/190°C. Roll the dough on a lightly floured surface to an approximately 12-inch circle. Ease into the tart pan without stretching, trim the edge, and chill for 15 minutes. Set the pan on a baking sheet.
  4. Line the shell with parchment and fill with pie weights. Bake for 15 minutes. Remove the paper and weights and bake for 8–10 minutes more, until the base looks dry and lightly colored. Reduce the oven to 350°F/175°C.
  5. Whisk the eggs, cream, and milk together. Spread the cooled onions and 1 cup (100 g) of the Gruyere in the shell. Pour in the custard and scatter the remaining cheese over it.
  6. Bake for 30–35 minutes, until the filling is set with only a slight central wobble and reaches 160°F/71°C in the center. If the crust darkens early, shield its edge with foil.
  7. Rest on a rack for 15 minutes before removing the pan’s outer ring and cutting into six wedges.

A useful detail

The total time includes chilling the pastry and resting the finished tart; the onion cooking overlaps the first chill.

Make it your own

  • Use an equal weight of Emmental instead of Gruyere.
  • For a leek version, replace half the prepared onion weight with cleaned, thinly sliced leeks; keep the same slow cooking method.

Keep it well

Refrigerate within 2 hours, or 1 hour above 90°F/32°C, at 40°F/4°C or colder for up to 3 days. Reheat covered wedges to 165°F/74°C, then uncover briefly to crisp the pastry. Keep chilled for transport.