In a large mixing bowl place Horseradish Sauce and mayonnaise and mix to combine, set aside.
Heat oil in a large heavy bottomed skillet over medium-high to high heat. Once shimmering add potatoes in a single layer. Don’t overcrowd the pan or your potatoes won’t crisp, if necessary fry in batches.
Leave the potatoes alone for a minute. Shake the pan, the potatoes should move freely. Shake the pan every minute or so to keep the potatoes from sticking. After about 5 to 7 minutes, once nice and brown on one side flip and repeat the process for another 5 to 7 minutes to brown the second side.
Add the onions the last 2 to 3 minutes.
Transfer hot potatoes and onions to the bowl with Horseradish Sauce. Toss to coat and season to taste with salt and pepper.
Serve warm garnished with chopped fresh parsley leaves.