Trim the radishes and slice them crosswise into 1/3-inch-thick rounds. Trim and discard the stems from a small handful of the tops, wash the leaves thoroughly, pat dry, and then finely chop enough to measure 2 Tbs. (Save the rest of the tops for another use.)
In a 10-inch skillet, melt the butter over medium heat. Add the radishes and cook, stirring occasionally, until they begin to soften, 5 to 7 minutes. Add the broth. Bring to a simmer, cover, and cook until the radishes are crisp-tender, 5 to 7 minutes. Uncover, raise the heat to high, and add the vinegar, sugar, and 1/2 tsp. salt. Cook, stirring occasionally, until the liquid is reduced to a glaze, 2 to 3 minutes. Garnish with the chopped leaves and serve.