Heat oil and butter in a large saucepan over medium heat. Add the onions and all the spices and herbs and cook on low heat for about 10 minutes, stirring, do not burn the onions.
Add the stock and passata, bring to the boil then add the frozen veg and potato. Bring back to the boil. Boil for 10 minutes, then reduce heat and simmer until veg are soft, about 1 hour. Add creme fraiche or cream to taste and serve.
You can put the soup into 4 or 5 small plastic containers and freeze until using.