Melt butter in olive oil over medium heat

Cook and stir onion, carrot, celery, and salt until vegetables are softened and onion is translucent, about 10 minutes

Reduce heat to low and add thyme sprigs and pepper

Stir in lentils until well coated

Add chicken stock and bring to a gentle simmer

Reduce heat to low, cover and cook, checking occasionally, until the lentils are tender and have absorbed all the liquid, about 35 minutes

Remove from heat and discard thyme stems

Stir in champagne vinegar and parsley

Season with salt and pepper to taste