Add 2 tablespoons of the olive oil to a Dutch oven or large, heavy-bottom pot over medium heat. Add the onions and sauté for 10 minutes, stirring occasionally (reduce to medium-low if onions start browning). Add 1 tablespoon of olive oil, salt and pepper, celery, and carrots. Sauté for 5 more minutes. Stir in the garlic, thyme, tomato paste, and Dijon, and cook for 1 minute. Add the bay leaves, chicken stock, and lentils. Stir and bring to a boil, then reduce the to a simmer. Cook uncovered until the lentils are cooked through, about 45 minutes to 1 hour.

If too much liquid cooks off, add more chicken stock, or some water. When adding more liquid, the zing of the Dijon and vinegar may taste diluted. If so, whisk one teaspoon of Dijon mustard into a few spoonfuls of broth in a small bowl, then add back to the broth.

Taste the soup to check the doneness of the lentils, and add more salt or pepper as needed. Remove the bay leaves. Add the kielbasa, spinach, and red wine vinegar, and cook for 2-3 more minutes to warm the kielbasa and wilt the spinach. Top with Parmesan before serving.