In a large pot of salted water, bring potatoes to a boil. Reduce heat to a simmer, and simmer for about 20-25 minutes, or until tender. Drain.
Combine potatoes, red onion, green onion, celery, parsley, pickle and apple cider vinegar.
To prepare the dressing, heat olive oil in a medium skillet over medium high heat. Add yellow onion and saute for about 3-4 minutes, or until tender. Add beer, vinegar, sugar, salt and pepper. Bring to a boil, reduce to a simmer, and simmer until reduced to about 1 cup.
Place dressing in a blender or food processor. Add mustard and process until smooth. Slowly add remaining olive oil and continue processing until smooth.
Pour dressing over potato salad and toss gently to coat well.