2To make the melted onions, heat the olive oil in a large skillet over medium heat and sauté the onions until translucent, about 7 minutes. Add the vinegars and a pinch of white pepper. Decrease the heat, stir in the sugar, and cook until the liquid is almost completely reduced and the onions have “melted” into a jam, 20 to 25 minutes. Remove from the heat. Stir in the thyme. Set aside.
3Place the puff pastry on a floured surface and roll out 1 inch larger all around. Lay on a baking sheet lined with parchment. Cut 3/4-inch-wide strips off each side of the puff pastry. Brush the edges of the pastry with the yolk mixture and place the cut strips on the edges to make a pastry rim. Cover the inside evenly with the toppings, starting with the melted onions and ending with the anchovies. Brush the rim with the yolk mixture. Place on a grate in the oven and bake until golden brown, about 15 minutes.
