My first experience was in Cleveland at Jonathon Sawyer's The Greenhouse Tavern (PS If you don't know Sawyer, who was just named to Food & Wine's 2010 Best New Chefs, pay attention, he's got skillz):
Next up, they surfaced in Washington, DC at Wes Morton's menu at Againn (with some intensely flavorful local butter - would you look at that color!):
I decided to try my hand at making them at home after finding them at the Penn Quarter FreshFarm Market. I had two choices of types of radishes and I think either would have worked, but I picked the ones that would have greater surface space to hold the butter:
Fresh Market Radishes with Butter & Sea Salt:
Radishes, cleaned and sliced to give flat surface
Butter, softened to room temperature
Herbs (I used parsley picked fresh from my windowbox herb garden)
1) Spread butter across top of the flat part of the radish. You'll want to fully cover the surface (don't get chintzy, even though this is healthy, its gonna be substantial butter).
2) Sprinkle a pinch of sea salt and herbs across the tops of the butter.
3) You could eat right away, or pop in the refrigerator until you are ready. I like to refrigerate because the butter firms up a bit more which makes it easier to eat. Enjoy!
UPDATE: April 17, 2010
I like to think I started a trend. Spotted this vendor at the Arlington Farmers Market... get your radishes before they run out!