Set a rack in the center of the oven and heat the oven to 450 degrees F.

Finely grate the zest from the lemon, preferably with a rasp-style grater. In a small bowl, combine the zest with 2 tablespoons of the oil. Cut the lemon in half and set aside.

On one side of a heavy, rimmed baking sheet, toss the asparagus with the remaining 1 tablespoon of the oil, 3/4 teaspoon salt, and 1/2 teaspoon black pepper. Spread out the asparagus in one layer. Lightly coat the other side of the baking sheet with oil, and place the salmon fillets skin side down on the oiled area. Sprinkle with 1 teaspoon salt and 1/2 teaspoon pepper. Roast until the asparagus are tender and the salmon is cooked to your liking (cut into a fillet with a paring knife to check), 10 to 13 minutes for medium. Keep in mind that the fish will continue to cook a bit more after it comes out of the oven. If the asparagus needs more time to cook, transfer the salmon to a platter, tent it loosely with foil, and return the asparagus to the oven until tender.

Arrange the salmon and asparagus on a platter and drizzle the lemon oil all over both. Squeeze a little juice from a lemon half over the salmon and serve immediately.