Trim off the tough ends from the asparagus and place on a small rimmed baking sheet.
Whisk together shallot, vinegar, 1 teaspoon lemon zest, lemon juice and 1/2 teaspoon salt in a small bowl. Let it sit for 10 minutes, then whisk in the olive oil until the dressing is emulsified. Taste and season with additional salt, if needed.
Pour 2 tablespoon of the vinaigrette over the asparagus and season to taste with black pepper. Sprinkle with 1 tablespoon water.
Slice the green onion into thin rings. Combine the green onion, parsley and remaining lemon zest in a small bowl.
Place the baking sheet in the oven and roast 10 - 12 minutes, until the asparagus is beginning to brown at the tips.
Transfer asparagus to a serving plate. Drizzle with the remaining vinaigrette and sprinkle with the parsley mixture. Finish the dish with a pinch of flaky sea salt, if you like.