Boil the potatoes in salted water until tender, 6 to 8 minutes. Remove from the heat and immediately transfer the potatoes to a bowl of ice water to stop the cooking. Cut the cooked potatoes into 1/2-inch thick coins and set aside.
Cut the bacon into 1/2-inch pieces. In a heavy-bottomed saute pan set over medium heat, cook the bacon until crispy, about 4 minutes. Remove the bacon to a paper towel-lined plate and reserve. In the same pan, add the onions
and cook until soft, about 2 minutes. Add the garlic and cook until fragrant, about 1 minute more. Stir in the mustard
and vinegar and scrape any browned bits from the bottom of the pan into the dressing
. Remove from the heat, gently stir in the potatoes and toss to coat. Stir in the parsley
and season with salt and pepper. Serve warm.