Gently fry the onions and parsnips in the oil on a low heat for about 15 minutes.
Add the garlic and apples and cook for a further 3-4 minutes, stirring regularly.
Add the stock and bring to the boil. Reduce the heat to a simmer and cook for about 15 minutes, until the parsnips are soft.
Remove from the heat and season with salt and freshly ground black pepper.
Blend the mixture in a food processor until smooth.
Stir in the milk, adding a little extra if required. Season to taste with salt and freshly ground black pepper.