Place the cubed potatoes in a large pot and cover with cold, salted water. Bring to a boil over high heat, then reduce heat and simmer until fork-tender, about 15-20 minutes. Drain well.

While potatoes are cooking, blanch the trimmed green beans in boiling water for 3-4 minutes until bright green and crisp-tender. Immediately transfer them to an ice bath to stop cooking, then drain thoroughly.

While the potatoes are still slightly warm, transfer them to a large mixing bowl. Gently fold in the cooked green beans, diced red onion, and chopped hard-boiled eggs.

In a small bowl, whisk together the mayonnaise, Dijon mustard, apple cider vinegar, celery seed, salt, and pepper until well combined and smooth. This is the dressing.

Pour the dressing over the potato and green bean mixture. Gently fold everything together until evenly coated. Be careful not to mash the potatoes.

Taste and adjust seasoning if necessary. Cover the bowl and refrigerate for at least 2 hours to allow the flavors to meld. Before serving, sprinkle with crumbled bacon.