Place the peeled and chopped potatoes into a large pot and cover with cold water. Add a generous pinch of salt and bring to a boil over high heat. Cook until the potatoes are just tender when pierced with a fork, about 15-20 minutes.
Drain the potatoes well and spread them out on a baking sheet or large plate to cool slightly for about 10 minutes; they should be warm but not hot to prevent sogginess.
While the potatoes cool, thinly slice the red onion, then rinse under cold water and pat dry to mellow their sharpness. Finely chop the celery and herbs, and measure out the mayonnaise, mustard, and vinegar.
Gently fold the mayonnaise, mustard, and vinegar into a bowl, stirring until smooth and combined. Taste and season with salt and pepper as needed.
Add the slightly cooled potatoes to a large mixing bowl. Gently mash some of the potatoes with a fork to break them up, but leave some chunks for texture.
Fold in the sliced red onion, chopped celery, and herbs, mixing gently to evenly distribute the ingredients without breaking down the potatoes.
Pour the dressing over the potato mixture and fold carefully until everything is coated and just slightly sticky. Adjust seasoning with more salt, pepper, or lemon juice if desired.
Cover the bowl and refrigerate the potato salad for at least 30 minutes to let flavors meld and the texture set.
Before serving, give the salad a gentle stir and taste again, adjusting seasonings if needed. Serve chilled or at room temperature, garnished with extra herbs if desired.
