Toss asparagus, mushrooms and scallions with next 4 ingredients (1/4 t. salt). Roast for 5 minutes on a large baking sheet.
Push veggies to each side of the baking sheet and place the fish in the center of the sheet. Season fish with remaining salt, pepper and lemon juice. Roast for about 10 minutes longer or until fish flakes with a fork and veggies are tender.