In a dutch oven (or large heavy-bottomed pot) heat 2 tablespoons of olive oil or duck fat until shimmering. Add the carrots and onions and cook until the vegetables begin to soften but not brown (about 10-12 min). Add the garlic and cook for 1-2 minutes more, until the garlic is fragrant.
Meanwhile, make the Bouquet Garni by tying together the rosemary, thyme, parsley and bay leaf with string
Once the vegetables are soft, add the lentils, stock, potatoes and the bouquet garni. Cook uncovered for 45 minutes, stirring occasionally. When ready, the lentils should be soft and most of the liquid should be absorbed. If the liquid is absorbed before the lentils are tender, add more stock (or water) 1/4 cup at a time until they’re ready.
When the lentils are finished, remove the bouquet garni. If the spinach leaves are larger, roughly chop them (or just add them is small) and stir into the lentils a bit at a time until they are all wilted. Cook for 4-5 minutes more.
Season the lentils well with salt and pepper, a good glug of olive oil (3-4 tablespoons) and 3-4 tablespoons of red wine vinegar. Stir everything one last time and check the flavors again. The lentils should have a little vinegary tang to them but not be sour.
To serve, spoon the lentils onto plates and swirl in a spoonful of sour cream or Crème fraîche (if using).