Start by cleaning the King oyster mushrooms. Remove any dirt with a damp cloth and chop them into small pieces.
In a large pot, add the chopped mushrooms and pour in the vegan chicken broth. Bring to a boil, then reduce to a simmer. Allow the mushrooms to cook for about 10-15 minutes or until tender.
Once cooked, drain the mushrooms and let them cool for a few minutes.
In a mixing bowl, combine vegan mayo, Dijon mustard, and dill. Stir well until fully blended.
While the mushrooms are cooling, chop the celery, slice the scallion, and dice the apple.
In the mixing bowl with the dressing, add the cooled mushrooms, chopped celery, scallion, and diced apple. Gently fold until well coated.
Add salt and pepper to taste. Adjust the seasoning to your preference.
For the best flavor, let the salad chill in the refrigerator for at least 30 minutes before serving.