Preheat oven to 450F. In a medium bowl, combine the oil, parsley (if using), garlic, salt and pepper. Quarter the potatoes. Cut the carrots on the diagonal into 1/2 inch slices. Cut the scallions (white and tender green parts) into 2-inch sections.
If the mushrooms are large, halve them. Scatter the potatoes, carrots, scallions and mushrooms over the bottom oa a shallow 1 1/2 quart baking dish. Drizzle 2 Tbsp. of the garlic oil over them and toss gently to coat. Place the dish in the oven and roast until the potatoes are almost done and beginning to brown, about 30 minutes; Stir the vegetables once or twice to ensure even cooking.
Meanwhile, cut the salmon into 1 1/2 inch cubes. Add the salmon to the bowl of garlic oil and toss gently to coat. Lower the oven temperature to 375F. Gently stir the vegetables in the baking dish, scatter the salmon on top and roast until the vegetables are tender and the salmon just flakes when tested with a fork, about 10 minutes longer.