Cook the unpeeled potatoes in boiling water until tender. Drain and let cool a bit until you can handle them. Slice the potatoes into medium-sized slices.
In a large bowl, blend the mustard, salt, pepper, herbs, garlic, vinegar and oil.
Add the potatoes, onions and parsley and toss gently to combine.
Cover and chill for at least 3 hours before serving for the best flavor!