Heat a thick bottomed pot over medium heat. Fry the onion in the olive oil until softened, but not coloured, 8-10 minutes. Stir regularly to ensure it doesn't catch.
Add the celeriac, potato and carrot. Mix well. Add the orange juice, lemon juice, sugar and water, plus plenty of salt and pepper. Bring to the boil, then turn the heat down to low. Cover and leave to simmer until the vegetables are completely soft, around 40-50 minutes, but depending on the size of your chunks. Stir a couple of times during this time, adding more water if necessary.
Take off the heat and mix in the chopped celeriac leaves (or flat leaf parsley). Season to taste with salt and pepper and add more lemon juice if desired. Leave for another 5-10 minutes before serving.
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