In skillet cook bacon till crisp; drain, reserving drippings. Crumble bacon. Cook onion in reserved drippings till tender. Combine bacon, cooked onion, worcestershire, salt, mustard and oregano. Add ground beef; mix well.
Shape into four 1 cm thick patties. Broil 8 cm from heat till desired doneness, turning once (allow about 10 minutes total time for medium). Combine mayonnaise and milk. Top each burger with 1 tomato slice and a few zucchini slices. Dollop burgers with mayonnaise mixture; sprinkle with parmesan cheese. Broil about 1 minute longer. Serve on buns or rolls. Makes 4 servings.