Start by peeling the potatoes and then cutting them into bit size pieces. Once done put them In a pot of water and cook on a medium high heat until tender. While potatoes are cooking chop the onion into small pieces and set aside.When potatoes are done let them cool for 30 minutes or so before adding in the other ingredients.Then when the potatoes are cool add in the mayonnaise, mustard, onion, salt and pepper and give that a good mix. Taste for seasoning and put in fridge for a few hours to chill or overnight which is better.