Heat the olive oil in a medium pot over medium heat. Add the onion, carrot, and celery. Saute, stirring frequently, for 5-7 minutes, or until the carrots are tender and the onion is soft and clear. Add the garlic and cook for 2 more minutes, stirring constantly.
Add the tomato paste, lentils, broth, bay leaf, thyme, salt, and pepper to the pot. Bring the mixture to a boil. Reduce the heat to low, cover, and simmer the soup for 45 minutes.
5 minutes before the soup is ready, you can add the sherry and finish simmering the soup uncovered. You can also add baby spinach or chopped greens, stir them into the soup, and simmer uncovered for the final 5 minutes of cooking time.
Add the vinegar or lemon juice to the soup. Taste and adjust seasonings as needed. If you’d like the soup to be thinner, add a bit of water to the pot. Serve the French lentil soup with a slice of toast and some cashew parmesan cheese, if desired.
Notes
Leftover soup will keep in an airtight container in the fridge for up to 5 days. It can be frozen for up to 1 month.