Bring a large pot of water to a boil and salt generously. Add potatoes to water and return to a boil, then lower heat to a simmer. Cook potatoes just until fork tender and drain in a colander. Let set for 3-4 minutes. Transfer the still hot potatoes to a large mixing bowl. Stir in the thinly sliced white onion parts and drizzle the apple cider vinegar over the top and gently toss. Cover with plastic wrap and put into the refrigerator to chill completely before proceeding.

When potatoes are cold through and through, remove the plastic wrap and add the crumbled bacon (reserving 2 tablespoons for garnish), chopped hard boiled egg, mayo, horseradish, and thinly sliced onion greens (reserving 2 tablespoons for garnish). Gently toss to combine and adjust taste to suit. Cover tightly and refrigerate for at least 2 hour, but preferably overnight before serving. Before serving, stir well and taste, adjusting taste to suit. Garnish with reserved thinly sliced onion greens and crumbled bacon.

Can be stored in refrigerator for 3 to 4 days.