Melt the butter together with the olive oil in a large, heavy-based saucepan over a medium heat. Sweat the onion, celery, carrot and bay leaf for 6-7 minutes, stirring occasionally. Add the gammon and continue to cook for 2-3 minutes, stirring a few times, before stirring in the lentils.
Cover with the stock, bring to the boil over a moderate heat, then reduce to a simmer. Simmer for 20-25 minutes until the lentils are tender; add water to the saucepan if the soup looks too dry.
Adjust the seasoning as necessary and discard the bay leaf. Spoon into ceramic soup bowls, garnish with a swirl of crème fraiche and a sprig of parsley. Serve immediately.
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