Bring a large pot of water to a boil. Add the potatoes and cook over medium-high heat until fork-tender, about 12–20 minutes depending on their size. Remove from heat and let them cool for 5–10 minutes so they’re easier to slice and handle.

Slice each potato in half. Using a small spoon, scoop out a bit from the center of each half and place the scooped potato into a medium bowl. To make enough filling, also scoop out all of the centers from 4–5 extra halves.

Mash the potato centers with a fork.

Add the mayonnaise, mustard, salt, and pepper, then mash until the mixture is smooth and creamy.

Spoon the filling into a piping bag, snip off the tip, and pipe it back into the center of each potato half.

Sprinkle with fresh chopped parsley and serve.