Caramelized Onions:

Melt the butter in a large saucepan.

Add the sliced onions and cook over low heat, tossing occasionally with a spatula, for 1 hour or until soft, sweet, and golden brown.

Transfer to a small bowl and refrigerate until cool to the touch, about 30 minutes.

Palmiers

Preheat oven to 400° F (205° C). Generously butter a half sheet rimmed baking tray. Freeze until ready to use.

With a rolling pin, roll puff pastry to approximately 12 x 14 inches. Use a sharp chef’s knife to trim the edges to create an even rectangle.

With a pastry brush, brush pastry with balsamic vinegar. Arrange sliced brie over pastry, covering the entire surface. Scatter with the caramelized onions.

With a rolling pin, gently roll the brie and onions into the dough.

With the back of a knife, lightly mark the pastry widthwise to divide it into 6 equal parts, each measuring about 2 1/3 inches wide. Using the markings as a guide, fold the pastry inward, then fold it over itself again. Finally fold inward again, as though you’re closing a book.* When finished, you should have 1 thick strip of pastry, about 2 1/3 inches wide and 14 inches long. With your hands, press down gently to secure.

Place strip on prepared tray. Freeze 10 minutes.

With a sharp chef’s knife, slice into 1-inch pieces. You should have about 14 pieces total. Arrange on prepared tray, cut sides up, allowing 2-3 inches in between each palmier. Use the palm of your hand to flatten each slice to create the classic heart shape.

Freeze 20 minutes more.

Brush the outside edge of each palmier with the beaten egg.

Bake in preheated oven for 15 minutes. With a flat metal spatula, flip the palmiers and bake for 12-15 minutes more, or until flaky and deeply golden brown. Remove from tray immediately.  Transfer to a wire rack to cool.

Brie and onion palmiers are best enjoyed on the day they are made. Serve warm or at room temperature.