Add the chopped mushrooms to a saucepan with 1 teaspoon of salt and the bay leaves. Bring to a boil and cook for about 5 minutes, until tender. Drain well, discard the bay leaves, and let the mushrooms cool completely.
Peel the celery root and grate it using the large holes of a box grater. Place the grated celery root in a large mixing bowl.
Cut the ham into small, bite-size pieces and add it to the bowl.
Season the chicken thighs with some of the remaining salt and black pepper. Sear in a skillet over medium heat for about 8 to 10 minutes per side, until golden and fully cooked.
Let the chicken cool slightly, then cut it into bite-size pieces.
Add the chicken, cooled mushrooms, and chopped parsley to the bowl with the celery root and ham.
Add the mayonnaise and season with the remaining salt and black pepper to taste. Toss gently until everything is evenly coated.
Cover and refrigerate for at least 2 hours before serving.
Stir gently before serving and adjust the seasoning if needed.