For the mustard creme fraiche: Mix together the creme fraiche, whole-grain mustard, Dijon and seafood seasoning in a small bowl and refrigerate until ready to serve.
For the mini crab cakes: Pulse the bread in a food processor to make coarse breadcrumbs; transfer to a large bowl. Add the crab, mayonnaise, lemon zest, Worcestershire, seafood seasoning, cayenne, scallions, 1/4 teaspoon salt and a few grinds of pepper. Mix gently to combine. Add the egg and mix gently but thoroughly. Roll the mixture into about 15 golf ball-sized rounds, then arrange them on a foil-lined baking sheet and flatten them slightly. Refrigerate at least 30 minutes to set.
Melt the butter in a large nonstick skillet over medium heat. Cook the crab cakes, in batches if necessary, turning once, until golden brown on both sides and heated through, 3 to 4 minutes per side.
Arrange the crab cakes on a serving platter and serve with the mustard creme fraiche dolloped on top or in a small bowl on the side. Sprinkle with the chives and serve with lemon wedges.