Preheat oven to 500 degrees. Trim the radishes and wash the greens, pat dry.
In large ovenproof skillet (I like a cast iron pan), heat the oil until hot. Add the radishes, season with salt and pepper and cook until lightly browned in some spots. This will need stirring and should only take 2-3 minutes.
Transfer the skillet to the oven and roast the radishes for 15 minutes, until crisp-tender.
Return the skillet to the burner, removing carefully from the oven. Stir in the butter to coat the radishes. Add the radish greens and cook over moderate heat until they are wilted, about 2 minutes.
Add the lemon juice and season with salt. Serve immediately.