Heat oven to 400°F On a rimmed baking sheet, toss the potatoes, mushrooms, 1 tablespoon of the oil, and 1/2 teaspoon each salt and pepper. Roast, tossing once, until the potatoes begin to soften, about 20 minutes.
Push the vegetables to the edges and place the salmon in the center. Season with 1/4 teaspoon each salt and pepper. Roast until salmon is opaque throughout, the mushrooms are tender, adn teh potatoes are golden brown, 12 to 15 minutes.
Meanwhile, in a bowl, whis the vinegar, mustard, honey, parsley, the remaining 2 tablespoons of oil, and 1/4 teaspoon each salt and pepper. Drizzle over salmon and vegetables before serving.