Start by boiling the sliced potatoes with salt until tender, then drain, reserving some of the cooking water.
Blend together olive oil, parsley, green onions, lemon juice, Dijon mustard, garlic, black pepper, and the reserved cooking water to create a vibrant herbed dressing.
Pour the herbed dressing over the warm potatoes, allowing them to absorb the flavors as they rest.
Add chopped celery and additional parsley and green onions to the dressed potatoes, then season with salt and pepper to taste.