Preheat the oven to 400 degrees. Carefully unroll the puff pastry onto parchment paper or a silicone mat. Use a rolling pin to roll it out into a roughly 10 inch by 16 inch rectangle. Poke the center of the pastry all over with a fork.
In a medium bowl, whisk together the ricotta cheese, 1/2 cup Parmesan, 1 egg yolk, lemon zest, lemon juice, 1/2 teaspoon Kosher salt, and pepper. Spread the ricotta mixture evenly over the top of the pastry, leaving a 1 inch border around the edge.
Layer the asparagus stems evenly over the ricotta, alternating stems and tips. Drizzle with olive oil, then sprinkle with flaky sea salt. Whisk the remaining egg white with a splash of water, then brush it around the border of the pastry.
Place in the oven and bake for 20-24 minutes, or until asparagus is tender and crust is golden brown. Sprinkle with remaining Parmesan and a little extra lemon zest, if desired, and serve.