How-to Make Red Radish Rosettes
With a small paring knife, slice off the stem so the radish rests on the cutting board with its root standing straight up. Cut a V into the top of the radish, removing the root. Cut two more V's into the top of the radish, positioning them like an X over the first V. You should end up with six pointed peaks.
Carefully cut an inverted V under each of the pointed peaks. Soak radishes in cold water for at least 8 hours, allowing petals to open.