Place the diced potatoes into a large pot and fill with enough water to cover the potatoes completely.
Bring the water to a boil, and cook for about 10 minutes, or until the potatoes are easily pierced with a fork. You don't want them to start falling apart when pierced, however.
Drain the potatoes in a colander and set aside.
Place the bacon and onions in a large skillet over medium-high heat. Fry until the onions are soft and translucent and the bacon is cooked.
Whisk together the vinegar, sugar, salt, mustard and pepper. Taste test and adjust accordingly. ( make it sweeter or more mustard, etc.)
Add the liquid to the skillet and bring to a boil. Add in the potatoes and parsley.
Heat completely through.
Spoon into a serving dish or bowl and serve.
Adjust the amount of sugar in this to your preference. Mike liked it tarter, I liked it slightly sweeter. It's up to your personal preference. Also adjust the whole grain mustard to your own liking. I LOVE the stuff and will use more next time.