Turn the broiler to high; put the rack 4 inches from the heat. Cut 4 tablespoons of the butter into bits. Or if it’s softened, just unwrap for dabbing. Trim the tough bottoms from the asparagus. Cut the salmon into 4 portions.
Put the salmon and asparagus on a rimmed baking sheet, skin side up, dab with bits of butter, and sprinkle with salt and pepper.
Broil undisturbed for 5 minutes. Carefully turn the salmon and shake the pan to roll the asparagus. Return the baking sheet to the broiler and cook until the salmon is cooked as you like and the asparagus is tender and browned in places. (No more than 5 additional minutes for medium to medium-well salmon, and less if you like your salmon closer to medium-rare.)
While the salmon and asparagus cook, put the remaining 2 tablespoons butter in a medium skillet over medium-high heat. When it’s hot, add the bread crumbs, sprinkle with salt and pepper, and stir gently to coat the crumbs with the butter. Cook, stirring frequently and adjusting the heat to avoid burning, until the crumbs are golden and crisp, 3 to 5 minutes. Cut the lemon into wedges.
When the salmon and asparagus are done, sprinkle the bread crumbs over the top. Serve with the lemon wedges.