Put the sweet potatoes in a large saucepan of salted water. Bring to the boil, reduce the heat and simmer for 5 minutes, then drain.
Heat the butter and oil in a large frying pan over a low heat until the butter has melted. Add the sweet potatoes, garlic and thyme. Cook over a medium-low heat for 10-15 minutes, turning the potatoes over after 5 minutes or so, a