In a small saucepan over moderate heat combine vinegar, sugar, Worcestershire and pepper. Allow sugar to dissolve in vinegar and liquids to come to a bubble. Remove sauce from heat and whisk in tomato sauce, then extra-virgin olive oil. Let dressing stand.
Arrange sliced tomatoes and onions on a serving platter. Season tomatoes and onions with salt, to taste Pour dressing over the tomatoes and onions and garnish with chopped parsley.