Eating apples give a sweet note to this root vegetable soup, topped with crisp pieces of bacon and crème fraîche.
Peel the apples, cut them into quarters and remove the cores. Cut the apples into thick slices. Melt 25g/1oz of the butter in a large saucepan and fry the apple pieces over a medium heat for five minutes, or until lightly browned, turning regularly. Using a slotted spoon, transfer the apple pieces to a plate and return the pan to the heat.
Add the remaining butter and oil to the pan and as soon as the butter melts, gently fry the onions, celeriac and carrot for 15 minutes, or until the onions are softened and very lightly browned, stirring occasionally. Add the garlic, apples, potatoes, thyme and bay leaf and cook for three minutes more, stirring.
Pour the stock into the pan, season with black pepper and bring to the boil. Reduce the heat to a simmer and cook for about 25 minutes, or until the celeriac and carrots are very soft, stirring occasionally.
Remove the pan from the heat. Remove the thyme stalks and bay leaf and discard. Cool slightly and then blend the soup with a stick blender until very smooth. (For an even more velvety soup, pass it through a fine sieve into a clean bowl.) Add a little extra water if necessary until the right consistency is reached. Alternatively, allow the soup to cool for a while and then blend in a food processor until smooth and return to the pan.
Adjust the seasoning with salt and pepper to taste. Set aside until ready to serve.
To make the garnish, brush the oil over the inside of a small non-stick frying pan and fry the bacon for 2-3 minutes on each side over a medium-high heat until browned and crisp. Drain on kitchen paper.
Mix the crème fraîche with the milk in a small bowl until smooth.
Reheat the soup gently just before serving, stirring constantly. Ladle into warmed bowls.
Drizzle the crème fraîche mixture into each bowl of soup. Carefully snip the crisp bacon into pieces with kitchen scissors and scatter on top. Sprinkle with the parsley, season with a little freshly ground black pepper and serve.