In a large stockpot, heat olive oil and sauté onion and celery for about 5 minutes, or until it starts to brown. Add garlic and sauté for 1 minute more.

Remove sausage from casings, and crumble into pot. Cook, stirring well to break up sausage and combine with onion mixture, until sausage is no longer pink.

Add hot pepper flakes, thyme, oregano and fennel seed to pot and sauté for another 1 minute.

Add the chicken broth, tomatoes with juice, spinach, lentils and bay leaves to the stockpot.

Bring soup to a boil, and then reduce heat to low, cover and let simmer for 45 minutes to an hour, or until lentils are soft when you bite into one.

When lentils are cooked, remove the bay leaves and discard.

Remove about ⅓ of the soup, and use an immersion blender or regular blender to puree this soup. Add pureed soup back into the pot and stir to combine.

Simmer soup, uncovered, for another 15 minutes.

Add salt and pepper to taste.