Preheat your oven to 375°F and line a baking sheet with parchment paper. While the oven heats, trim the woody ends from the asparagus (about 1-2 inches from the bottom), then cut any particularly thick spears in half lengthwise so they cook evenly. Grate your parmesan cheese fresh if you haven't already—I find this gives a much sharper, more complex flavor compared to pre-grated cheese. Measure out your salt, pepper, lemon zest, and garlic powder into a small bowl and set aside.

On a lightly floured surface, unfold your puff pastry sheet and gently roll it into a 10x12 inch rectangle. Transfer it to your prepared baking sheet. Using the tines of a fork, prick the entire surface of the pastry to prevent it from puffing up unevenly. With a sharp knife, score a border about 1 inch from all four edges—don't cut all the way through, just create a guide line where the pastry will rise up and frame your filling.

In a small bowl, whisk together the egg and 1 tablespoon of water to create an egg wash. Brush this mixture along the scored border line—this will help the border puff up beautifully and turn golden brown during baking. Be careful not to let the egg wash drip down the sides, as it can prevent the pastry from rising properly.

Spread the boursin cheese evenly over the center of the pastry, staying within the scored border line. The boursin provides a rich, garlicky base, so you don't need to add extra garlic powder here. Sprinkle the spice mixture from Step 1 (salt, lemon zest, garlic powder, and black pepper) evenly over the cheese. Scatter the grated parmesan on top, then arrange your trimmed asparagus spears in parallel lines or a crisscross pattern over the filling—I like to arrange them neatly because they'll be visible and look beautiful when the tart is finished.

Place the assembled tart in your preheated 375°F oven and bake for 28-32 minutes, until the pastry border is puffed and golden brown and the asparagus tips are tender. The exact timing depends on your oven and how thick your asparagus spears are, so start checking around the 25-minute mark. The pastry should be deep golden, not pale, for the best flavor and crispness.

Remove the tart from the oven and let it cool for 2-3 minutes on the baking sheet so the pastry sets slightly and becomes easier to handle. Using a sharp knife, slice along the scored border lines to portion the tart into rectangles or squares. Serve while still warm—the pastry will be at its crispiest and the cheese will still be creamy.