Preheat the oven to 425 degrees. Unless very small, halve the radishes lengthwise from stem to root end. Toss them with the oil on a large rimmed baking sheet. Roast, turning once or twice, until the radishes are tender and beginning to brown, about 25 minutes. Remove the radishes from the oven (but do not turn it off).
Drizzle the soy sauce over the radishes and sprinkle with the scallions. Toss well and continue roasting for 5 minutes longer. Turn the radishes and any juices on the baking sheet into a serving bowl. Sprinkle with the sesame seeds and serve immediately.
Recipe courtesy of Jack Bishop's VEGETABLES EVERY DAY (HarperCollins, 2001)