Preheat grill to medium-high.
Place fish and asparagus on a large rimmed baking sheet and coat both sides with cooking spray; sprinkle with 1/2 teaspoon salt, pepper and garlic powder.
Oil the grill rack (see Tips). Place the asparagus on one side, perpendicular to the grates; place the fish on the other side. Grill the fish, turning once, until opaque, 3 to 5 minutes per side (depending on thickness); grill the asparagus, turning occasionally, until lightly charred, 5 to 7 minutes.
Place butter, lemon juice and the pinch of salt in a small, microwave-safe bowl. Microwave on High to melt the butter, about 25 seconds. Drizzle each portion of fish and asparagus with about 1 tablespoon of the lemon butter.