If using farmed salmon, broil it for 4 to 5 minutes.
Preheat broiler with rack 4 inches from heating element. Stir together soy sauce, lemon juice, sugar, and ginger in a small bowl until sugar is dissolved.
In a large straight-sided ovenproof skillet, toss asparagus and mushrooms with oil, season lightly with salt and pepper, and spread in an even layer. Broil 5 minutes. Add soy-sauce mixture, stir to coat, and broil 3 minutes more.
Add salmon, sprinkle with salt, and drizzle with oil and some sauce from pan. Broil 2 to 3 minutes more (for medium-rare). Serve, with lemon wedges.