Preheat oven to 400°F (200°C). Position one rack in the center.
Rinse asparagus under cool water. Pat dry with a clean kitchen towel.
Hold one asparagus spear at each end and bend until it snaps. Trim the woody ends with a knife and discard.
Place the trimmed asparagus on a large, rimmed baking sheet. Drizzle with olive oil.
Sprinkle sea salt and freshly ground black pepper over the asparagus.
Evenly distribute the small pats of butter over the asparagus. Squeeze half of the lemon juice over the asparagus.
Roast for 10 minutes. Remove from oven and gently toss the asparagus.
Sprinkle the Parmesan cheese over the asparagus and return to the oven for an additional 2 minutes. Finish under the broiler for 1–2 minutes, until the Parmesan turns golden and bubbly. Watch closely to avoid burning.
Remove from oven and immediately squeeze the remaining lemon juice over the top before serving. Garnish with flaky sea salt, if desired, and serve warm.