In a small saucepan, combine soy sauce, lemon juice, vegetable oil, sugar, and sesame oil. Bring to a boil, then let cool to room temperature.
Transfer soy mixture to a medium bowl and stir in mushrooms. Let stand, stirring occasionally, until mushrooms soften, about 3 to 5 minutes. Add chicken, cabbage, radishes, carrot, celery, and mint; toss to combine. Top salad with scallion.