Wash the beets and boil them for 30–40 minutes until fork-tender. Let cool, peel, and cube.
Boil potatoes with skin on for 15–20 minutes until tender but firm. Cool slightly, then cube.
In a small bowl, whisk together olive oil, apple cider vinegar, Dijon mustard, salt, and black pepper.
In a large bowl, gently combine potatoes, beets, and red onion.
Pour dressing over the vegetables and toss carefully to coat evenly.
Refrigerate for at least 30 minutes before serving to allow flavors to develop.
Garnish with fresh parsley if desired and serve chilled.