2 tablespoons olive oil
1 onion, quartered
2 garlic cloves
4 1/2 pounds sweet potatoes, peeled and roughly chopped
leaves from a sprig of fresh thyme
1 vegetable stock cube
flaky salt and freshly ground white pepper
Heat oil in a large saucepan. Add onion and garlic and cook over moderate heat until softened and glossy but not browned. Add the sweet potatoes and herbs, and toss with the onion, garlic and oil. Cover with cold water, add the stock cube and some pepper and boil until all ingredients are soft and ready to blend. Remove from heat and process with a hand blender until smooth. Season to taste.