Heat oven to 200C/180C fan/gas 6. Put the quail's eggs in a bowl of warm tap water to get them used to the change in temperature. Then bring about an inch of water to the boil in a saucepan and prepare a bowl with some iced water. Gently lower the quail's eggs into the boiling water and boil for 2 mins, then drop them into the iced water. Leave them in there until chilled. Carefully peel and set aside.
Toss the celeriac, apple, herbs and cheese together with the pumpkin seeds, chilli flakes and season. Combine the egg yolk with the Dijon and stir into the celeriac mixture. Whisk the egg white to stiff peaks and fold into the celeriac fritter mix.
Heat a little rapeseed oil and butter in a non-stick frying pan. Put two dessertspoonfuls of fritter into the hot fat and fry for 1 min or until a caramelised crust has formed. Turn carefully and cook for a further 2 mins. Place on a lined baking sheet and bake for 5 mins. Remove from the oven and allow to settle on the sheet for a couple of mins. Dress the parsley and red onion with a little of the vinegar and rapeseed oil. Divide between plates and top with the quail's eggs and celery salt. Serve with the parsley salad.