In a small bowl, whisk together 6 tablespoons red wine vinegar, 2 tablespoons sugar and 3/4 teaspoon salt; add 1/2 cup finely chopped red onions and let stand for 1 hour. Halve 9 hard-boiled eggs lengthwise. Transfer the yolks to a bowl and mash with 5 tablespoons mayonnaise, 1 1/2 teaspoons juice from the pickled onions, 1 teaspoon dijon mustard and 1/4 teaspoon salt. Fill each egg white half with the yolk mixture. Sprinkle the pickled onion and black pepper on top.