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.