Heat the oven to 200C/180C fan/gas 6 and lightly oil a 25 x 20cm baking dish or roasting tin.
Heat 2 tbsp of the oil in a large frying pan over a medium heat. Fry the onions until lightly golden and soft, about 10 mins. Remove from the pan and set aside.
Put the grated potato and celeriac in a large microwaveable bowl. Add a tablespoon of water and cook on high for 1 min (or steam in a tiered steamer for 2-3 mins).
Drain off any moisture from the veg and add the fried onions, 3 tbsp rapeseed oil, the beaten egg and parsley. Season with 1 tsp salt and some black pepper. Stir well to combine and spoon into the prepared dish. Press down lightly to compact the vegetables and brush the remaining tablespoon of oil over the top.
Bake for 45 mins-1 hr until the top is golden brown and crisp and the centre soft. Scatter over a little more parsley to serve
