Heat oven to 400° F. On a rimmed baking sheet, toss the potatoes, mushrooms, 1 tablespoon of the oil, and ½ teaspoon each salt and pepper.
Roast, tossing once, until the potatoes begin to soften, about 20 minutes.
Push the vegetables to the edges of the pan and place the salmon in the center. Season with ¼ teaspoon each salt and pepper.
Roast until the salmon is opaque throughout, the mushrooms are tender, and the potatoes are golden brown, 12 to 15 minutes.
Meanwhile, in a bowl, whisk together the vinegar, mustard, honey, and parsley, the remaining 2 tablespoons of oil, and ¼ teaspoon each salt and pepper. Drizzle over the salmon and vegetables before serving.