Preheat oven to 450°F. Trim radishes; reserve greens. Cut radishes in half lengthwise. Wash and dry greens, discarding any yellowed or wilted leaves; set aside.
Toss radishes with olive oil, pepper, and ½ teaspoon of the salt in a 12-inch ovenproof skillet; arrange cut side-down. Roast in preheated oven until starting to soften, 10 minutes.
Open oven, and quickly scatter butter, thyme, and garlic over top. Continue roasting until radishes are tender, 10 to 12 minutes. Remove from oven, add vinegar, radish greens, and remaining ¼ teaspoon salt; stir until greens are wilted. Transfer to a serving bowl; serve hot.
