Heat oil and butter in heavy-based pan; add onion and salt, saute for about 8 minutes or until onion is very soft.
Add sweet potatoes and sweat over low heat for 6 minutes.
Pour over enough chicken stock to just cover sweet potatoes, cover with a cartouche and simmer very gently for about 40 minutes or until very tender (by this stage most of liquid should have been absorbed).
Remove from heat, stand 5 minutes.
Drain off any remaining liquid. Set aside.
Puree sweet potato with extra butter and extra oil until smooth and light. Check seasoning.
Add some of the reserved liquid if necessary to give the desired consistency.
