Preheat the oven to 400°F and line a baking sheet with parchment.

Roll the puff pastry out slightly on a floured surface to smooth the seams, then transfer to the sheet.

Score a border 1/2 inch in from the edge on all sides, cutting only halfway through the dough.

Prick the center area inside the border all over with a fork.

Brush the border with egg wash.

Spread the Dijon mustard evenly over the center, staying inside the border.

Scatter the Gruyere over the mustard.

Toss the asparagus with olive oil, salt and pepper, then lay the spears in a single layer across the cheese, alternating tip direction.

Bake 22 to 25 minutes, until the border is puffed and deep golden and the asparagus is tender when pierced with a knife.

Slice into squares or strips and serve warm or at room temperature.