Begin by placing the whole unpeeled potatoes into a large pot and covering them with salted water by 1 inch. Heat over medium-high until boiling, then cook until the potatoes are just tender, about 15 to 25 minutes, depending on size. Drain the potatoes and cut them into 1 1/2-inch pieces while still warm. Transfer to a large mixing bowl.

In the meantime, cook the bacon in a large skillet over medium-high heat until crispy. Remove the bacon with a slotted spoon and set it on a paper-towel-lined plate.

Add the sliced shallots to the skillet with the bacon fat and cook for about a minute, until they begin to crisp. Remove from heat.

In a separate bowl, combine mustard, red wine vinegar, salt, and pepper. Mix in the shallots and bacon fat. Crumble the bacon into the mixture. Toss the warm potatoes with the dressing, adjusting the seasoning as needed. Garnish with chopped parsley and additional vinegar if desired. Serve warm or at room temperature.