Preheat oven to 375°F (190°C). Line 2 large baking sheets with parchment paper.

In a large bowl, toss asparagus with 1 tablespoon of olive oil, herbes de provence, salt, and pepper.

Heat the remaining 1 tablespoon of olive oil in a skillet over medium-low heat.

Sauté thinly sliced onions until browned and very tender, about 20 minutes. Set aside.

On a lightly floured surface, roll out one sheet of puff pastry to form a 10x15 inch rectangle.

Trim a 3-inch wide strip off the bottom and right sides of the pastry to form a 7x12 inch rectangle. Save the pastry strips.

Cut the 7x12 inch rectangle into 3 even 4x7 inch pieces.

Brush a 1/2 inch border around the edge of each rectangle with beaten egg.

Cut the trimmed pastry strips into 6 1/2-inch pieces and use them to form a crust around the edge of each rectangle.

Repeat the process with the second sheet of puff pastry, creating 6 rectangular crusts in total.

Divide the sautéed onions down the center of each pastry.

Neatly arrange the asparagus stalks on each pastry, on top of the onions, with the tips all facing the same direction.

Bake at 375°F (190°C) until the pastry is puffed and golden brown, approximately 25-30 minutes.

Remove the tarts from the oven and top with shaved Gruyere cheese and fresh ground pepper, if desired.

Let cool slightly before serving.