Pat salmon pieces dry and season with 1/2 teaspoon salt and pepper. Heat oil in a 12-inch nonstick or well-seasoned skillet over medium-high heat.
Arrange salmon in one layer and cook for 3 to 4 minutes without moving, then turn carefully and cook for 1 to 2 minutes more until the thickest pieces reach 145°F. Transfer to a clean plate.
Add asparagus, water and remaining salt to the skillet. Cover and steam over medium heat for 3 to 4 minutes until crisp-tender, then uncover and evaporate remaining water for 1 minute.
Lower heat to low, add butter and garlic and stir around the asparagus for 30 to 45 seconds. Stir in lemon juice and zest.
Return salmon gently and warm for 1 minute, spooning the lemon butter over the fish without breaking it apart. Divide salmon and asparagus evenly among 4 plates.