Preheat oven to 400°F.
Whisk 2 tablespoons vinegar, 1 tablespoon oil, 1 tablespoon soy sauce, the grated garlic, 1 teaspoon mustard and ¼ teaspoon each crushed red pepper, salt and pepper together in a large bowl.
Add salmon, asparagus and red onion; toss to coat.
Spread the salmon mixture evenly on a large rimmed baking sheet; roast until the salmon is just cooked through and the asparagus is tender-crisp, 12 to 14 minutes.
Serve with lemon wedges, if desired.
