Place a rack in top third of oven; preheat to 400°. Arrange asparagus on one side of a large rimmed baking sheet and onion wedges on the other. Drizzle vegetables with oil; season with salt and toss to coat. Spread vegetables in an even layer, keeping each on their own side of baking sheet. Roast until just softened, about 10 minutes.
Meanwhile, season salmon with 1 teaspoon salt and ¼ teaspoon pepper. In a small bowl, mix butter, garlic, parsley, lemon zest, and 3 tablespoons lemon juice; season with salt and pepper.
Remove baking sheet from oven; push onion to one side and asparagus to the other. Arrange salmon skin side down in center of baking sheet. Spoon about three-quarters of butter mixture over salmon. Spoon remaining butter over vegetables.
Bake salmon 8 minutes. Turn on broiler and broil on high, watching closely, until salmon is just cooked through and vegetables are tender and browned in spots, 3 to 5 minutes more.
Top with parsley. Serve with lemon wedges alongside.
