Add chopped potatoes to a large pot of salted water. Cover and bring to a boil. Boil potatoes until fork-tender, about 10 minutes.
While the potatoes are cooking, sauté the onion, celery, and bell pepper in white wine vinegar with a pinch of salt until just softened. Turn off heat and set aside.
For the dressing, combine the Dijon, crème fraîche, olive oil, coriander, paprika, fennel seed, and cayenne in a medium bowl. Stir together well.
Strain the potatoes and add them to a large bowl along with the onion, bell-pepper mixture, scallions, parsley, cilantro, Dijon dressing, and a generous shower of salt and black pepper. Mix well. Add in chopped egg and gently fold into the potatoes.
To serve, add a sprinkle of cayenne on top, fennel fronds, or more parsley.
Can be made ahead and refrigerated. Stir together before serving.