Bring a pot of generously salted water to a boil. Add potatoes and cook until soft, approx 10 minutes. Drain.
While the potatoes are boiling, combine red onion, green pepper, mustard, olive oil, vinegar and mayonnaise and season with salt and pepper.
Once the potatoes are done, toss them into the bowl with dressing and add scallions and additional salt and pepper to taste.
Serve room temperature or refrigerate until ready to serve.