Place spinach in a small bowl. Drizzle with vinaigrette and toss to coat; set aside.
In a large bowl, combine the egg, chopped spinach, cheeses, onion, ketchup, Worcestershire sauce, salt and pepper. Crumble beef over mixture and mix well. Shape into four patties.
Grill burgers, covered, over medium heat or broil 4 in. from the heat for 5-7 minutes on each side or until a meat thermometer reads 160° and juices run clear. Top with mozzarella cheese. Grill 1-2 minutes longer or until cheese is melted. Serve on buns with spinach mixture. Yield: 4 servings.