In a medium bowl combine eggs, milk, parsley, dry mustard, salt, Worchestershire sauce, and pepper. Beat with a whisk until well mixed and evenly colored; set aside.
In a large skillet heat butter over medium heat until melted. Add mushrooms and the 1 tablespoon chopped onion; cook for 2 minutes, stirring occasionally. Pour egg mixture into skillet. Cook over medium heat, without stirring, until mixture begins to set on the bottom and around the edges.
Using a spatula or large spoon, lift and fold the partially cooked egg mixture so the uncooked portion flows underneath. Continue cooking for 2 to 3 minutes or until egg mixture is cooked through but is still glossy and moist. Remove from heat. Serve immediately. If desired, sprinkle with green onions.