For the soup: ， Olive oil: 1 tablespoon， Butter: 1 tablespoon， Onion, finely chopped: 1， Leeks, chopped: 3 (1 pound)， Garlic, finely chopped: 2 cloves， Chicken stock: 8 cups， Large potatoes, peeled and cubed: 4， Kosher salt and freshly ground black pepper: ， Truffle oil, for drizzling (opt): ， For the fried parsley: ， Veg oil, for deep-frying: 0.5 cup， Fresh flat-leaf parsley: 6
1In a large saucepan or slow cooker, that the oil and butter over medium-low heat. Add the onion, leeks, and garlic and cook gently, stirring often, until soft but not coloured. This will take a good 15 to 20 mins. Then add the stock and the potatoes, season with salt and pepper, and cook until the potatoes are soft, about 15 to 20 mins.
2While the potatoes are cooking, heat the vegetable oil in a small sauté pan until it is almost smoking. Fry the parsley springs in the hot oil for 30 seconds to 1 minute until they are crispy. Using a slotted spoon, remove the parsley and set aside on paper towels to drain excess oil.
3When the potatoes are cooked, remove the soup from the heat, let cool slightly, and then blend with an immersion blender until smooth. Take care not to splash yourself-the soup will be very hot. Taste and adjust the seasoning. Reheat gently, if needed.
4Serve in bowls, topping each with a sprig of fried parsley. Drizzle with the truffle oil, of using.
Leeks always seem to have a bit of mud in them. To clean, cut them in half lengthwise, starting halfway up the white portion and slicing all the way through the green ends. Fill a large jug with cold water and put in the leeks, green end down. Swish then around, then drain. Repeat three or four times to be sure you've removed all the grit.