Prep: 20 mins
| Cook: 16 mins
| Extra time: 1 hour, soaking
Place eggs in a saucepan, cover with cold water and bring to a boil over medium-high heat. When water boils, remove from heat and cover the saucepan. Let the eggs sit for 16 minutes, then drain and cool eggs in a bowl of ice water. Peel and halve lengthwise.
Pickle the onions by whisking the vinegar, sugar, and 3/4 teaspoon salt together in a mixing bowl. Stir in the onions, cover, and let the onions marinate in the mixture for 1 hour.
Scoop yolks out from split hard-boiled eggs. In a bowl, combine yolks with mayonnaise, pickle juice, mustard, and remaining salt; use a fork to smash the mixture together until smooth.
Pipe or carefully spoon the mashed yolk mixture back into the egg whites. Garnish with pickled onion, and sprinkle with black pepper.