Caramelized Onion Gruyere Tart (Print View)

Sweet caramelized onions and Gruyere cheese baked in buttery puff pastry for elegant meals.

# Components:

→ Pastry

01 - 1 9-inch round sheet of puff pastry, thawed if frozen

→ 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 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 1/2 cups grated Gruyere cheese
16 - 1 tablespoon Dijon mustard

# Directions:

01 - Set oven temperature to 375°F and allow to fully preheat.
02 - Roll out puff pastry and fit into a 9-inch tart pan with removable bottom. Trim excess pastry from edges. Prick base with a fork to prevent puffing. Refrigerate while preparing remaining components.
03 - In a large skillet over medium-low heat, melt butter with olive oil. Add sliced onions, salt, pepper, and sugar. Cook with frequent stirring for 30-35 minutes until onions achieve deep golden color and soft texture. Add thyme during final 5 minutes. Remove from heat and cool slightly.
04 - In a mixing bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper until fully combined and smooth.
05 - Brush interior base of chilled pastry shell with Dijon mustard. Distribute caramelized onions evenly across base. Sprinkle grated Gruyere cheese uniformly over onions. Pour custard mixture over cheese and onions, distributing evenly.
06 - Place tart in preheated oven and bake for 35-40 minutes until filling is set in center and top surface achieves golden-brown coloring.
07 - Remove from oven and allow to cool for 10 minutes before slicing and plating.

# Expert Advice:

01 -
  • The filling is creamy and custardy without being heavy, letting those sweet onions shine.
  • You can prep the caramelized onions a day ahead, making this feel less daunting than it actually is.
  • It's fancy enough to serve guests but honest enough to eat alone with a simple salad and no regrets.
02 -
  • If you hear sizzling when the custard hits the pastry, your pan wasn't hot enough—use a preheated baking sheet on the oven rack to ensure even cooking.
  • Don't walk away from the onions for more than five minutes at a time; caramelization needs attention and stirring, not a watched pot situation.
  • The tart will continue to cook slightly after you remove it from the oven, so pull it out when the very center still wobbles just a hair.
03 -
  • Brush the raw pastry shell with mustard immediately after placing it in the pan—the thin, even coating prevents sogginess better than any other step.
  • If your tart puffs up too much during baking, gently press the center back down with a wooden spoon after 20 minutes; it'll settle.
  • Leftover tart keeps beautifully in the fridge for three days and is actually wonderful cold or gently reheated in a 300°F (150°C) oven for 10 minutes.
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