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Caramelized Onion Tartlets

These savory caramelized onion tartlets combine a crisp, buttery pastry shell with a creamy filling of slow-cooked yellow onions, fresh thyme, garlic, eggs, cream, and nutty Gruyère cheese. The onions are cooked gradually until deeply golden, creating a rich sweetness that complements the flaky crust and melted cheese.
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Course: Appetizer
Cuisine: European
Keyword: Dough Chilling Time
Cook Time: 25 minutes
Dough Chilling Time: 30 minutes
Total Time: 55 minutes

Ingredients

For the Pastry Dough

  • 1 cup chilled unsalted butter cut into small cubes
  • 1/4 cup cornstarch
  • 3/4 cup ice-cold water with additional water available if required
  • 2 1/4 cups all-purpose flour plus extra for dusting the work surface
  • 3/4 teaspoon salt

For the Caramelized Onion Filling

  • 4 ounces Gruyère cheese shredded
  • 1 garlic clove crushed
  • 2 large eggs
  • 3 medium yellow onions sliced thinly and evenly
  • 1/4 cup heavy cream
  • 1 teaspoon fresh thyme finely chopped
  • 2 tablespoons unsalted butter
  • 4 teaspoons cornstarch blended with 1/4 cup cold water
  • 1/2 teaspoon freshly ground black pepper
  • 1/2 teaspoon salt

Instructions

  • Combine the all-purpose flour, 1/4 cup cornstarch, and 3/4 teaspoon salt in a large mixing bowl, whisking until the dry ingredients are evenly distributed.
  • Add the cold, cubed unsalted butter. Work it into the flour mixture with a pastry cutter or fork until the texture resembles coarse crumbs.
  • Pour in the ice water gradually, approximately 2 tablespoons at a time, mixing gently after each addition. Continue only until the dough holds together when pressed and can be formed into a ball. Add a little additional ice water only when necessary.
  • Shape the dough into a flat disk and wrap it securely in plastic wrap. Refrigerate for at least 30 minutes and up to 1 hour. The wrapped dough may also be refrigerated for as long as 3 days.
  • Before rolling, allow the refrigerated dough to rest at room temperature for approximately 15 minutes so it becomes easier to handle.
  • Melt 2 tablespoons of unsalted butter in a large, heavy-bottomed skillet over medium heat.
  • Add the thinly sliced yellow onions and cook for approximately 35–45 minutes, stirring frequently, until they become deeply golden and thoroughly caramelized.
  • If the onions begin browning too rapidly or appear likely to burn, lower the heat or add a small splash of water to the skillet.
  • Once the onions are fully caramelized, stir in 1/2 teaspoon salt, 1/2 teaspoon black pepper, the crushed garlic, and finely chopped fresh thyme.
  • Continue cooking for 1 additional minute, stirring to distribute the seasonings evenly. Remove the skillet from the heat and allow the onion mixture to cool.
  • In a large bowl, whisk together the eggs and heavy cream.
  • In a separate small container, combine 4 teaspoons cornstarch with 1/4 cup cold water, stirring until completely smooth. Whisk this slurry into the egg and cream mixture.
  • Add the cooled caramelized onion mixture and whisk or stir until evenly incorporated.
  • Fold in the shredded Gruyère cheese and mix thoroughly.
  • Preheat the oven to 375°F.
  • Lightly coat a clean work surface with all-purpose flour. Place the chilled pastry dough on the prepared surface and roll it into an approximately 14-inch circle.
  • Cut rounds from the pastry with a 3-inch circular cookie cutter or a glass of comparable diameter.
  • Collect the remaining pastry scraps, gently roll them out again, and cut additional rounds as necessary.
  • Carefully press each pastry circle into an individual well of a mini muffin pan, forming small tartlet shells.
  • Spoon approximately 1 tablespoon of the caramelized onion filling into each pastry shell.
  • Transfer the filled tartlets to the preheated oven and bake for 25–30 minutes, or until the pastry has developed a golden color and the centers are completely set.
  • Allow the tartlets to cool in the pan until they are comfortable to handle. Carefully remove each tartlet from the mini muffin tray.
  • Serve the caramelized onion tartlets warm or at room temperature.
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Notes

Make certain the caramelized onion mixture has cooled before combining it with the eggs. Adding hot onions could begin cooking the eggs prematurely.
Keeping the butter and water very cold while preparing the pastry helps preserve a flaky crust.
If the onions begin to scorch during caramelization, reduce the temperature or introduce a small splash of water rather than allowing them to burn.
For additional richness, a small amount of extra grated Gruyère may be sprinkled over the tartlets immediately after they are removed from the oven.