Ingredients:

  • 2 tbsp (30ml) olive oil
  • 2 lbs (900g) yellow onions, thinly sliced (about 4 large)
  • 1 tsp (5g) packed brown sugar
  • 1/4 tsp (1.5g) dried thyme
  • 1/4 cup (60ml) dry red wine (or balsamic vinegar), optional
  • 1/4 tsp (1.5g) salt, or to taste
  • 1/4 tsp (1.5g) black pepper, or to taste
  • 8 sheets phyllo dough, thawed (about 1/2 of a standard 1-lb package)
  • 6 tbsp (85g) unsalted butter, melted
  • 1/2 cup (50g) Gruyère cheese, grated
  • Optional: Fresh thyme leaves, for garnish

Instructions:

  1. Heat olive oil in a large skillet over medium heat. Add onions, brown sugar, thyme, salt, and pepper. Cook, stirring occasionally, until onions are softened and deeply caramelized, about 30-40 minutes. Reduce heat as needed to prevent burning. If using, deglaze pan with red wine (or balsamic vinegar) during the last 5 minutes of cooking, scraping up any browned bits. Remove from heat and cool slightly.
  2. Preheat oven to 375°F (190°C). Lightly brush the tart pan with melted butter.
  3. Place one sheet of phyllo dough in the tart pan, allowing the edges to overhang. Brush with melted butter. Repeat with remaining phyllo sheets, rotating each sheet slightly to cover the entire pan and brushing each layer with butter.
  4. Spread the caramelized onions evenly over the phyllo crust. Sprinkle with Gruyère cheese.
  5. Bake for 35-40 minutes, or until the phyllo crust is golden brown and crispy and the cheese is melted and bubbly.
  6. Let the tart cool in the pan for 5-10 minutes before carefully removing it. Garnish with fresh thyme leaves, if desired. Slice and serve warm or at room temperature.