In a large bowl, combine the ground beef, 2 tablespoons dressing mix, and ¼ cup sour cream; mix well (the mixture will be soft). Form the mixture into 4 burgers. Grill the burgers, turning once, until done (10 to 12 minutes); set aside. In a small bowl, combine the remaining dressing mix and remaining sour cream; mix well. Place each grilled burger on a bun. Top each burger with a lettuce leaf and ¼th of the diced tomatoes and onions. Place a dollop of dressing on top of each burger before serving.