In a large saucepan, cook the bacon lardons over a medium heat until they begin to crisp and release their smoky fat.
Add the butter or oil if needed, then stir in the onion and celery. Cook gently for 5 minutes until soft and translucent.
Add the garlic, carrots and diced swede. Cook for another 3 to 4 minutes so the vegetables begin to soften.
Stir in the red lentils and bay leaf, then pour over the chicken stock.
Bring to a gentle simmer and cook for 25 to 30 minutes, stirring occasionally, until the lentils have softened and the soup has thickened.
Remove the bay leaf, then stir through the cream if using, and most of the chopped parsley.
Taste and season well with salt and black pepper.
Serve hot with a final scattering of parsley and a thick slice of buttered bread.
