Ingredients:

  • 1 ¼ cups (150g) all-purpose flour, plus extra for dusting
  • ½ teaspoon (2.5ml) salt
  • ½ cup (113g) cold unsalted butter, cut into cubes
  • ¼ cup (60ml) ice water
  • 2 tablespoons (30ml) olive oil
  • 3 large yellow onions, thinly sliced (about 750g or 1.6 lbs)
  • 1 teaspoon (5ml) balsamic vinegar
  • ½ teaspoon (2.5ml) dried thyme
  • Salt and freshly ground black pepper to taste
  • 1 cup (115g) shredded Gruyère cheese
  • 2 large eggs
  • ½ cup (120ml) heavy cream (double cream)
  • Pinch of nutmeg

Instructions:

  1. Prepare the Pastry: Combine flour and salt. Cut in the cold butter until the mixture resembles coarse crumbs. Gradually add ice water until the dough comes together. Form into a disc, wrap in plastic wrap, and chill for at least 30 minutes.
  2. Caramelize the Onions: Heat olive oil in a large skillet. Add sliced onions and cook over medium-low heat, stirring occasionally, until deeply golden brown and caramelized (about 45 minutes - 1 hour). Stir in balsamic vinegar and thyme; season with salt and pepper.
  3. Preheat and Roll the Dough: Preheat oven to 375°F (190°C). On a lightly floured surface, roll out the chilled dough to a 12-inch circle.
  4. Assemble the Tart: Gently transfer the dough into the tart pan, pressing it into the bottom and up the sides. Trim any excess dough.
  5. Add the Cheese and Onions: Sprinkle half of the Gruyère cheese evenly over the bottom of the crust. Spread the caramelized onions evenly over the cheese. Top with the remaining Gruyère.
  6. Prepare the Egg Mixture: In a bowl, whisk together eggs, heavy cream, and nutmeg. Season with salt and pepper.
  7. Bake the Tart: Pour the egg mixture evenly over the onions and cheese. Bake for 45-55 minutes, or until the crust is golden brown and the filling is set. Let cool slightly before serving.