Cook the chopped carrots, celery, and onions in a Dutch oven over medium heat with a couple of tablespoons of olive oil. Cook for about 10 minutes or until the vegetables have softened.
Make the bouquet garni by tying together the celery thyme, parsley, and bay leaf with twine.
Once the vegetables are ready, add the lentils, the stock, the diced potatoes, and add the bouquet garni.
Occasionally stir the lentils, keeping an eye on them and stirring to prevent from burning. Cook until the lentils are soft and most of the liquid is absorbed, about 45 minutes.
When the lentils are finished, remove the bouquet garni, roughly chop the spinach, and stir into the lentils cooking everything for a couple of minutes more.
Season the lentils with salt and pepper, a drizzle of olive oil, and 1-2 tablespoons red wine vinegar. Stir again to mix together.
Serve warm with a little of creme fraiche spooned into the dish.