Yes, I have done it with just youghurt, but only if I am just making a small batch that will be eaten straight away. I find that the youghurt does not adhere to the potatoes if it is kept for more than a day in the fridge. No, I have not added mustard, but I will try it. Oh, if you go with youghurt, use a palin greek, good quality one. Hope it works for you.