Preheat the oven to 400°F. Arrange a rack in the middle of the oven.

Make the lemon caper butter: Place the butter, capers, and lemon zest in a small bowl. Mash with a fork to combine.

Prepare the asparagus: Arrange the asparagus on a rimmed baking sheet. Drizzle with olive oil, season with ½ teaspoon salt and several grinds of pepper, and toss to coat. Spread out in a single layer around the perimeter of the baking sheet, making room for the salmon in the center.

Prepare the salmon: Season the salmon fillets all over with ½ teaspoon salt and several grinds of pepper. Place skin-side down (if you bought them with the skin on), spaced evenly apart, in the middle of the baking sheet, pushing the asparagus to the side if needed. Divide the lemon-caper butter among the fillets, doing your best to spread it evenly. Don’t worry too much since the butter will melt in the oven.

Roast: Roast until the asparagus is crisp-tender and the salmon is just cooked through. An instant-read thermometer inserted into the thickest part of the salmon registers 120°F to 130°F for medium-rare, 10 to 15 minutes.

Drizzle with lemon juice and serve: Drizzle lemon juice over the salmon and asparagus and garnish with a few more grinds of pepper before serving.