Preheat oven to 350°F.

In a small bowl, stir together salt, ash, and water to make a paste that holds together when you squeeze it with your hand (the amount of water will depend on how dense the ash is).

Cover the celery root with the paste, pressing it down so that it adheres. Place the celery root in a baking dish. Bake until you can easily insert a small knife, about 2 hours.

Just before the celery root comes out of the oven, melt the butter in a small pan over low heat, and stir in the parsley. Set aside.

Remove celery root from the oven and place on a serving dish. Serve whole at the table and crack the crust with a meat mallet or the heel of a knife. Use a spoon to scoop out the celeriac and drizzle the parsley butter over the top. Celery root can also be sliced and served in individual portions.