Ingredients:

  • 4 large Yellow Onions, thinly and evenly sliced (approx. 700 g)
  • 2 Tbsp Unsalted Butter
  • 1 Tbsp Olive Oil
  • 1 tsp Light Brown Sugar (Optional)
  • 1 Tbsp Balsamic Vinegar, good quality
  • 1 tsp Fresh Thyme Leaves, chopped
  • Kosher Salt & Black Pepper, To taste
  • 1 sheet (14 oz) All-Butter Puff Pastry, chilled
  • 4 oz Gruyère Cheese, finely grated (115 g)
  • 1 Large Egg, for the egg wash
  • 1 tsp Water, to mix with egg wash
  • 4 small Thyme Sprigs (Optional, for garnish)

Instructions:

  1. Melt the butter and oil together in a large, heavy-bottomed pan over medium-low heat. Add the sliced onions and a pinch of salt. Cook, stirring occasionally, for 10–15 minutes until they are fully wilted and translucent.
  2. Reduce the heat to the lowest setting possible. Continue cooking the onions, stirring every 5–10 minutes, for another 30–45 minutes until they turn a deep amber-brown, sticky, and drastically reduced in volume.
  3. Stir in the brown sugar (if using), balsamic vinegar, and fresh thyme. Cook for 2 minutes until the liquid has evaporated. Taste and adjust seasoning with salt and pepper.
  4. Transfer the onion mixture to a bowl and allow it to cool completely. Cooling for about 30 minutes in the fridge is ideal, as warm filling will melt the pastry.
  5. Preheat the oven to 400°F (200°C). Lightly grease the cavities of a 12-cup standard muffin tin. Unroll the chilled puff pastry and use a 2.5-inch (6 cm) round cutter to stamp out 12 circles.
  6. Gently press one pastry circle into each muffin cup. Whisk the egg and water together for an egg wash and brush the rim of the pastry edges lightly. Sprinkle a small layer of grated Gruyère (about 1 tsp) into the base of each cup. Top with a spoonful of the cooled onion mixture and finish with a final light sprinkle of Gruyère.
  7. Place the tartlets in the preheated oven. Bake for 15–18 minutes, or until the pastry is puffed up, golden brown, and the cheese is bubbly and lightly colored.
  8. Carefully remove the tin from the oven. Let the tartlets sit in the tin for 5 minutes before gently easing them out. Serve warm or at room temperature, optionally garnished with a tiny sprig of fresh thyme.