Peel and chop the celeriac into matchsticks or coarsely grate. Plunge the matchsticks into boiling water, drain and immediately cool in cold water. When cold drain well to remove all excess water.
In a large bowl mix together the mayonnaise, mustard, lemon juice and parsley.
Add the celeriac and again mix well, making sure all of the celeriac is coated in the sauce.
Line ramekins with clingfilm. Fill with the celeriac mix and cover the tops with clingfilm. Put weights on top, then place in the fridge to chill and set for 2 hours.
Carefully remove from the ramekin moulds. Serve with smoked mackerel fillets.