Heat oven to 400F. Unfold puff pastry on a parchment-lined baking sheet. If needed, gently roll to smooth creases and create an even 10x12 inch rectangle. Score a 0.5 inch border all around with a paring knife, cutting halfway through the pastry. Prick the inner rectangle all over with a fork. Refrigerate while you prep the filling.

Bring a large pot of salted water to boil. Add asparagus and cook 2 minutes until bright green and just tender. Immediately transfer to an ice bath to stop cooking, then drain and pat very dry. This keeps the asparagus crisp-tender and vivid green.

Whisk Dijon and heavy cream together. Spread evenly within the scored border of the pastry. Scatter half the Gruyere over the cream mixture.

Lay asparagus spears side by side across the tart, alternating direction for a pretty presentation. They can overlap slightly. Sprinkle with lemon zest, thyme leaves, salt, and pepper. Top with remaining Gruyere.

Brush the pastry border with egg wash. Bake 22-25 minutes, rotating the pan halfway through, until the pastry is deep golden and puffed and the cheese is bubbling and spotted with brown. The bottom should be crisp when you lift a corner with a spatula.

Let cool 5 minutes on the pan. Drizzle with olive oil, scatter thyme sprigs over top, and cut into squares or rectangles. Serve warm or at room temperature.