Pour the water into a small saucepan along with the wine, garlic, onion, and thyme. Bring to a boil over high heat, then season to taste with salt and pepper.
Add the radishes, cover, reduce heat to medium-low, and simmer until the radishes are just tender, 6 to 8 minutes.
Remove the radishes to a serving dish with a slotted spoon and keep warm. Bring the liquid to a boil over high heat, and add a pinch of sugar. Boil until the liquid has reduced by half, then pour over the hot radishes.