Firstly peel the parsnips and keeping one aside, chop and cook the rest in boiling water for 15-20 mins until soft. Drain well and mash the cooked parsnips and leave to cool. Keep the mash as dry as you can. (Don't be tempted to add butter or any liquid as it must be firm)
In a separate pan bring the 150mls water and butter to a rolling boil and very quickly beat in the plain flour. Remove from the heat. Keep beating until the dough forms a large, soft ball which comes away from the edge of the pan. Leave to cool for a minute or two and slowly beat in the eggs a little at a time.The mixture will slacken off a little but should be of a dropping, not pouring consistency.
Add the mustard powder and grated Parmesan cheese. Beat well. Season to taste with salt and pepper. Fold in the cooked parsnip mash, chopped parsley and grate in the whole parsnip to add some texture.
For the best results chill until the fritter mixture has firmed up. (It will keep in the fridge overnight) Drop spoonfuls of the dough into hot oil and fry for 3 to 5 minutes until golden and cooked through. Drain on kitchen paper and serve hot.