Make the celeriac mash by steaming the celeriac in a steamer (or in pan with a little water and the lid on for about 15 minutes) until tender.
Drain and roughly mash with the butter.
Preheat the oven to 180oC.
Fry the onions and/or leek, carrot, celery, fennel, dill and bay leaf in the ghee in a large pan over a medium heat for about 10 minutes until almost tender.
Add the wine, stir and simmer until the wine has evaporated.
Add 500ml hot water and bring to a simmer.
Add the fish, cover and poach for about 5 minutes until cooked through.
Remove the fish and place into an ovenproof dish.
Mix the arrowroot with a little cold water until smooth, then add it to the pan stirring constantly for a few minutes as the sauce thickens up.
Take off the heat, stir through the parsley and season.
Flake the fish in the dish and pour the veg sauce over it.
Cover with the roughly mashed celeriac and bake for 30 minutes to heat through.
Preheat the grill, cover the mash with the grated cheese and grill on high for 5 minutes until the top is golden.