Thaw fish, if frozen. Rinse fish and pat dry with paper towels. Measure the thickness of fish.
In a 10-inch skillet combine the broth, onion, pepper, and thyme or marjoram. Bring to boiling. Place fish in skillet, tucking under any thin edges so fish cooks evenly. Cover and simmer for 4 to 6 minutes per 1/2-inch thickness or until fish just flakes easily with a fork. Remove fish to a hot platter; keep warm.
Bring liquid in skillet to boiling; boil, uncovered, over medium-high heat for 3 to 5 minutes or until reduced to about 1/2 cup. Combine water and cornstarch; stir into liquid in skillet. Cook and stir until thickened and bubbly. Cook and stir 2 minutes more. Stir in salad dressing and parsley. Serve herb sauce and, if desired, lemon slices with fish. Makes 4 servings.