Potato and Onion Soup

1 Tbl olive oil
1 Tbl butter
1 large onion, peeled and chopped
4 cloves garlic, peeled
1 kg potatoes, peeled and roughly chopped
4 1/2 cups vegetable stock
1/2 cup sour cream
salt and pepper

Place oil and butter in a large saucepan over medium heat. When butter is melted, add onion and garlic and cook until softened but not coloured. Add potatoes and stock and bring to the boil. Simmer until potatoes are tender.
Puree mixture adding sour cream slowly. Season to taste.