How to Make It
Put the potatoes in a large pot of water and bring to a boil. Cook over moderate heat until tender, about 25 minutes. Drain and let cool completely, then cut into 1-inch pieces.
In a large bowl, combine the mayonnaise with the mustards, vinegar and olive oil and whisk until smooth. Add the onion, roasted red peppers and parsley. Gently fold in the potatoes and season with salt and pepper. Cover and refrigerate overnight.