Caramelized Onion Gruyere Tart (Printer-friendly)

Flaky tart with sweet caramelized onions and melted Gruyere on a buttery crust, perfect for elegant gatherings.

# What You’ll Need:

→ Pastry

01 - 1 sheet (9-inch round) puff pastry, thawed

→ Caramelized Onions

02 - 2 tablespoons unsalted butter
03 - 2 tablespoons olive oil
04 - 4 large yellow onions, thinly sliced
05 - 1 teaspoon salt
06 - 1/2 teaspoon freshly ground black pepper
07 - 1 teaspoon granulated sugar
08 - 2 teaspoons fresh thyme leaves or 1 teaspoon dried thyme

→ Filling

09 - 3 large eggs
10 - 1 cup heavy cream
11 - 1/2 cup whole milk
12 - 1/4 teaspoon ground nutmeg
13 - 1/2 teaspoon salt
14 - 1/4 teaspoon freshly ground black pepper
15 - 1.5 cups Gruyere cheese, grated
16 - 1 tablespoon Dijon mustard

# Directions:

01 - Set oven to 375°F and allow to reach temperature.
02 - Roll out puff pastry and fit into a 9-inch tart pan with removable bottom. Trim excess edges and prick base with fork. Refrigerate while preparing remaining components.
03 - Heat butter and olive oil in large skillet over medium-low heat. Add sliced onions, salt, pepper, and sugar. Cook, stirring frequently, for 30 to 35 minutes until onions achieve deep golden color and tender consistency. Add thyme in final 5 minutes. Transfer to bowl and cool slightly.
04 - In mixing bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper until fully combined and smooth.
05 - Brush interior of chilled pastry shell with Dijon mustard. Distribute caramelized onions evenly across base. Sprinkle grated Gruyere cheese over onions. Pour custard mixture evenly over cheese and onion layers.
06 - Place assembled tart in preheated oven for 35 to 40 minutes until custard filling is set and top achieves golden brown color.
07 - Remove from oven and allow to cool for 10 minutes before removing from pan. Slice and serve warm or at room temperature.

# Expert Tips:

01 -
  • Those caramelized onions taste like pure comfort wrapped in crispy pastry—sweet, savory, and utterly addictive.
  • You look like you spent hours in the kitchen when really you just let the oven do most of the work while you pour a glass of wine.
02 -
  • Don't rush the caramelization—I once tried to speed it up by turning up the heat, and the onions scorched instead of sweetening, creating a bitter tart I had to scrape into the bin.
  • If your filling doesn't set properly by the end of baking, it means the oven temperature wasn't quite right or you didn't bake long enough; use a thermometer next time and watch for that gentle jiggle in just the very center.
03 -
  • A splash of white wine added to the onions in the last few minutes of caramelization deepens their flavor with a subtle complexity that guests always notice but can't quite identify.
  • Make sure your custard mixture is completely smooth before pouring—any lumps of egg white will remain gritty in the finished tart, and a few extra seconds of whisking prevents this entirely.
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