Scrub potatoes well under running water. In a large pot, add scrubbed potatoes and cover with water. Bring to a boil and cook until fork tender – about 15 minutes. Drain the potatoes and let them cool until safe to touch. Dice the cooked potatoes into 1-inch dice. Season with apple cider vinegar, olive oil, salt and pepper while still warm. Let them cool completely in the fridge while you prepare the dressing.
In a bowl, combine the avocado with the mustard, lemon juice, salt and pepper. Mash with a fork until creamy and smooth. Add the green onions, celery and yellow pepper, and mix to combine.
Gently mix the avocado dressing into the cooled potatoes and adjust for salt and pepper if desired. Serve immediately!