Place potatoes in a large pot, cover with water and a lid and bring to a boil over high heat. Reduce heat to medium-high and cook for 30 minutes or until potatoes are easily pierced with a fork or knife.
Meanwhile, combine sour cream, mustard and vinegar in a small bowl. Stir until well combined, add a pinch of salt and taste to see if you need more cream or vinegar (or lemon juice, if using). Set aside.
When cooked, rinse potatoes under cold water, then slice into small pieces. Place potatoes in a bowl, along with the sour cream mixture. Stir until all potatoes are coated, taste to see if you want more salt.
Sprinkle with parsley and serve, or cover with plastic wrap and store in the fridge until serving (or until you cook the eggs). The salad tastes best right after it's made, but will last in the fridge for 1-2 days.