Restaurant-Authentic Caramelized Onion & Gruyère Puff Pastry Triangles with Thyme Oil & Balsamic Gastrique

Instructions

1. 1

   Mise en place: arrange all ingredients in small bowls and prep vessels before beginning. Mince garlic into paste with 1/4 teaspoon fleur de sel using knife. Grate Gruyère and Parmigiano-Reggiano separately. Combine thyme sprigs and rosemary sprig in small bowl. Measure vinegar and honey into separate containers.

2. 2

   Preheat convection oven to 375°F (or conventional to 400°F) and position rack to upper-middle position. Line heavy-bottomed baking sheet with parchment paper.

3. 3

   Prepare herb-infused oil: gently heat 3 tablespoons extra-virgin olive oil in small saucepan over low heat until it registers 160°F on instant-read thermometer. Remove from heat, add reserved thyme and rosemary sprigs, cover, and allow to steep for 30 minutes. Strain through fine mesh and reserve at room temperature.

4. 4

   Begin onion caramelization: melt 3 tablespoons butter in heavy-bottomed 12-inch skillet over medium-high heat, swirling until it foams and turns light hazelnut brown, about 2 minutes. Add julienned onions and stir continuously to coat with hot butter, then immediately reduce heat to medium-low.

5. 5

   Cook onions uncovered for 35-40 minutes, stirring every 5 minutes with wooden spoon, allowing them to develop a deep mahogany color while releasing their natural moisture. Scrape fond from pan bottom every 8 minutes to build caramelization layers.

6. 6

   When onions have achieved rich golden-brown color, increase heat to medium and add garlic paste, stirring constantly for 1 minute until raw garlic scent disappears.

7. 7

   Deglaze pan: pour white wine around the edges of the skillet, scraping all caramelized bits with wooden spoon. Allow liquid to reduce by half, about 3 minutes, concentrating the umami flavors.

8. 8

   Remove from heat and fold in Dijon mustard, thyme leaves (stripped from 4 remaining sprigs), and rosemary. Stir in warmed honey, then season with fleur de sel and cracked pepper. Taste and adjust seasoning. Cool to room temperature, about 15 minutes.

9. 9

   While onions cool, prepare balsamic gastrique: pour aged balsamic vinegar into small heavy-bottomed saucepan and reduce over medium heat until it coats the back of a spoon and measures about 3 tablespoons, approximately 8-10 minutes. Pour onto parchment to cool; it will thicken further as it cools.

10. 10

    Unfold thawed puff pastry on clean work surface and allow it to come to room temperature for 5 minutes. Using sharp knife and ruler, cut pastry into 12 equal rectangles (approximately 3x4 inches each).

11. 11

    Assemble pastries: lay all 12 rectangles on work surface. Place 1.5 tablespoons cooled onion filling in center of each rectangle, leaving ½-inch border on all sides. Top each portion of filling with ¾ ounce Gruyère cheese (divided evenly).

12. 12

    Prepare egg wash emulsion: whisk 2 egg yolks with 1 tablespoon water using fork until completely homogeneous. Using fine pastry brush, apply thin, even coat of egg wash around the ½-inch border of each rectangle.

13. 13

    Fold each rectangle diagonally to form triangle, then press edges firmly with fork tines, creating a decorative seal and preventing steam escape during baking.

14. 14

    Transfer sealed triangles to prepared baking sheet, spacing them 2½ inches apart. Brush entire top surface of each triangle with remaining egg wash using light, feathering strokes to ensure even coverage.

15. 15

    Create cheese-paprika finish: combine 2 tablespoons Parmigiano-Reggiano and ½ teaspoon smoked paprika in small bowl, then sprinkle mixture evenly across the tops of all pastries in a light, even layer.

16. 16

    Bake for 16-18 minutes until pastries achieve deep golden-brown color and puff slightly, rotating pan halfway through baking at the 8-minute mark for even browning.

17. 17

    Remove from oven and allow pastries to rest on hot baking sheet for 2 minutes (residual heat ensures pastry sets), then transfer to wire cooling rack.

18. 18

    Plate and finish: arrange 3 pastry triangles on warm appetizer plate. Drizzle 1 teaspoon cooled balsamic gastrique in artistic line across plate. Apply 2-3 small drops herb-infused oil around plate perimeter. Finish with 2-3 sea salt flakes scattered across top of pastries and plate edge. Serve immediately while pastry retains warmth and crispness.

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