Thaw fish, if frozen. Cut fish fillets into 4 serving-size portions. Rinse fish; pat dry with paper towels.
Place fish fillets in a 2-quart rectangular baking dish; sprinkle with lemon-pepper seasoning. Bake, uncovered, in a 450 degree F. oven until fish flakes easily with a fork (allow 4 to 6 minutes per 1/2-inch thickness).
Meanwhile, for mushroom sauce, in a small saucepan cook mushrooms and green onion in margarine or butter until tender. Stir together chicken broth, wine or chicken broth, and cornstarch; add to mushroom mixture all at once. Cook and stir until mixture is thickened and bubbly; cook and stir for 2 minutes more.
Transfer fish fillets to serving plates; spoon mushroom sauce over fish.