Preheat oven to 400º F. 

On a lightly floured surface roll out the pastry dough sheet to about a 12-inch square. Cut the pastry dough into 4 equal squares.

On a parchment lined baking sheet, drizzle a little olive oil in the center of each quadrant of the baking sheet. 

Drizzle a little balsamic glaze over the olive oil. Season with a pinch each of kosher salt, black pepper and fresh thyme.

Shingle the sliced onion over the seasoned oil and balsamic glaze. 

Sprinkle with a little grated Gruyere. 

Lay one slice of pastry dough over the cheese and onions. Lightly press down and all four sides. 

Brush the pastry dough with eggwash and bake for about 20 minutes. Until the pastry is puffed and golden.

Let cool slightly, then flip the onion tarts over and top with more grated Gruyere if desired.