Preheat oven to 400 degrees F (200 degrees C). Place a baking stone or baking sheet on the bottom rack.

Line another baking sheet with parchment paper.

Dust a work surface with flour.

Roll the puff pastry into a 10x15 inch rectangle.

Transfer the pastry to the prepared baking sheet.

Score a half-inch border around the edge of the pastry (do not cut all the way through).

Prick the pastry inside the border with a fork.

Brush the pastry with beaten egg.

Bake for 15-20 minutes, or until golden brown.

Cool on the baking sheet for a few minutes.

Press down any puffed-up areas with a measuring cup or glass.

Let cool completely.

Heat olive oil in a skillet over medium-low heat.

Add thinly sliced onions and a pinch of salt.

Cook, stirring occasionally, until onions are soft and golden brown (about 10 minutes).

Add apple cider vinegar and stir until absorbed.

Remove from heat and let cool completely.

Scatter shredded fontina cheese over the pastry within the border.

Top with caramelized onions, dried fig rounds, and coarsely chopped walnuts.

Sprinkle with crumbled feta cheese, fresh thyme leaves, salt, and pepper.

Drizzle with olive oil.

Return the tart to the oven on the preheated baking stone.

Bake for 10-15 minutes, or until the fontina cheese melts.

Drizzle generously with honey.

Let cool for a few minutes.

Cut into 16 squares and serve.