Position a rack in the center of the oven and heat the oven to 450°F. Line a large rimmed baking sheet with parchment.
In a medium bowl, toss the mushrooms, leek, oil, 1/2 tsp. salt, and 1/4 tsp. pepper.
Finely grate the lemon zest. In a small bowl, mix the zest with 1 tsp. salt and 1/4 tsp. pepper.
Arrange the salmon on the baking sheet, rub the lemon zest mixture on the tops and sides of the fillets, and scatter the mushrooms and leeks around the fish in a single layer.
Roast until the vegetables are tender and the fish is opaque in the center, about 15 minutes. If the fish finishes cooking before the vegetables, transfer it to a plate and continue to roast the vegetables until tender, 3 to 5 minutes more.
Combine the arugula and roasted vegetables in a large bowl. Squeeze about 1 Tbs. juice from the lemon onto the vegetables and toss to combine. Season to taste with salt, pepper, and more lemon juice. Divide the mixture among four serving plates. Top each salad with a piece of the fish and serve.
nutrition information (per serving): Calories (kcal): 410; Fat (g): 16; Fat Calories (kcal): 140; Saturated Fat (g): 1.5; Protein (g): 41; Monounsaturated Fat (g): 9; Carbohydrates (g): 8; Polyunsaturated Fat (g): 3.5; Sodium (mg): 530; Cholesterol (mg): 105; Fiber (g): 2;