Melt the butter and oil in a large saucepan. Gently fry the onions and parsnips for 15 minutes, or until the onions are softened. Add the garlic and apples and cook for a further two minutes, stirring regularly.
Pour over the stock and bring to the boil. Reduce the heat to a simmer and cook for about 20 minutes, or until the parsnips are very 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.
Add a little curry powder to the fried onions and parsnips before stirring in the stock for a delicious twist.