Remove the seasoning packet from the potatoes and cook according to package directions. Set aside to cool.
In a large bowl, whisk together the mustard, garlic and lemon juice along with 1/2 of the seasoning packet. Slowly whisk in the olive oil and season with a few cranks of freshly ground black pepper. Taste, adding salt if desired. I find the seasoning packet salty enough, but feel free to add in more!
Halve the potatoes, then add to the bowl alongside the parsley, scallions, radishes and celery. Toss to combine, season more as needed— add in more lemon juice for acid/brightness, more olive oil for richness and/or salt/pepper. Enjoy!