In a large pot, brown the sausage over medium heat, breaking it into crumbles. Remove and set aside.
In the same pot, sauté onion, celery, and carrots until tender, about 5 minutes.
Add garlic and sauté for 1 minute until fragrant.
Return sausage to the pot. Add lentils, chicken broth, diced tomatoes, basil, and oregano.
Bring to a boil, then reduce heat and simmer uncovered for 30 minutes or until lentils are tender.
Stir in spinach until wilted, season with salt and pepper.
Serve hot, garnished with grated Parmesan cheese.