Put the lentils with an unpeeled clove of garlic in a saucepan with cold water to cover. Bring to the boil and simmer for about 25 minutes or until tender.
Melt the butter and olive oil in a large saucepan. Cook the onions and the remaining, peeled garlic clove until soft. Add half the sorrel and cook for a minute until the sorrel wilts. Stir in ¾ of the lentils and the chicken stock. S