Preheat oven to 400°F. Line a rimmed sheet pan with foil and lightly oil it.
Whisk melted butter, lemon juice, zest, garlic, paprika, half the salt, and half the pepper.
Toss asparagus with olive oil and a pinch of the remaining salt and pepper. Spread on the pan.
Place salmon over asparagus, skin-side down. Brush fillets generously with the lemon-garlic butter.
Bake 12–14 minutes, until salmon flakes easily and reaches 125–130°F and asparagus is tender-crisp.
Spoon pan juices over salmon. Broil 1 minute for light browning, if desired.
Rest 3 minutes. Sprinkle with parsley and serve right away.