Peel 1 watermelon radish (about 12 oz.). If radish is wider than 3 inches, cut in half lengthwise. Slice crosswise as thinly as possible. Mix with 1/3 cup seasoned rice vinegar. Cover and chill at least 10 minutes or up to 1 hour, stirring occasionally. Sprinkle with 1 teaspoon black sesame seeds, if desired.