In a food processor fitted with a metal blade, blend together yogurt, anchovy paste, lemon juice, vinegar, Dijon mustard, Worcestershire sauce, garlic, 2 tablespoons of Parmesan cheese, and pepper to taste.
Cover and chill at least 2 hours and up to 2 days.
Wash and dry lettuce. Place in large salad bowl.
Sprinkle with red onion and remaining 2 tablespoons Parmesan cheese.