Boil potatoes in salted water until soft, approx 10 minutes. Drain and let cool. While the potatoes are boiling, combine red onion, green pepper, mustard, olive oil, vinegar and mayonnaise and season with salt and pepper. Mix well and let the flavors marinade while the potatoes cook. Once the potatoes are done and cool, mix into the bowl and add scallions and additional salt and pepper to taste. Serve room temperature or refrigerate until ready to serve.