Place the radishes in a large skillet and add just enough cold water to cover, about 2 1/2 cups. Add the butter, sugar, salt, and pepper and bring to a boil. Reduce heat to medium-low and simmer until the radishes are tender when pricked with a paring knife and the liquid has reduced to a glaze, about 12 minutes. If the radishes are tender but the liquid hasn’t reduced sufficiently, use a slotted spoon to transfer them to a serving dish and continue reducing the liquid. Spoon it over the radishes and serve with buttered crusty bread.
Tip: The long, slender icicle radish is typically available in spring and early summer. If you prefer the more common red radish, which is round, slice it in half vertically.