- Preheat oven to 450F.
- Heat olive oil in a sauté pan. Add green onion and sauté about 1 minute. Add asparagus and cook over medium-high heat until asparagus is crisp-tender, about 5 minutes. Remove from heat.
- Roll out pastry to a 10 x 16-inch rectangle on a lightly floured piece of parchment paper. Transfer pastry and paper to a baking sheet. Sprinkle gruyere evenly on pastry, leaving a 1-inch border around the edge. Spoon asparagus mixture evenly over gruyere.
- Sprinkle cheese over all. Sprinkle with salt and pepper.
- Bake until pastry is golden brown, 20 to 25 minutes. Let cool slightly and serve warm. Cut into 8 (4 x 5-inch) pieces.