Slice a 12-in. section of baguette in half lengthwise, cut into 2-in. pieces, and toast in a 350° oven until golden brown on edges.
Using scissors, snip nori into bits, then pulverize in a spice grinder (if you don't have one, snip into tiny pieces).
Mix nori powder with butter; smear thickly onto toasts. Top with radishes and radish greens.