Gently heat the oil and butter in a large saucepan, add the chopped onion and cook on a gentle heat for 10 minutes or until the onion is soft but not brown. Add the garlic and cook for a further 3 minutes.
Add 3/4 of the peas, the chopped mint leaves, and 3/4 stock. Cover the saucepan with a tight fitting lid and cook on a medium boil for 10 minutes.
Blend the soup in a food processor, you will have a thick puree. Return the puree to the pan, season with salt and pepper and add the remaining peas and stock. Cook for a further 5 minutes.
Serve with crusty, fresh bread and/or garlic croutons.