Heat olive oil in a large Dutch oven or soup pot over medium-high heat. Add sliced sausage and cook for 5-7 minutes until browned on both sides, breaking it into bite-sized pieces as needed.

Add diced onion, carrots, and celery to the pot with the sausage and stir well. Cook for 8-10 minutes, stirring occasionally, until vegetables become tender and the onion turns translucent and fragrant.

Stir in minced garlic and dried Italian seasoning, cooking for 1-2 minutes until aromatic. Add the can of diced tomatoes with their juices and stir to combine all ingredients.

Pour in the dried lentils and chicken broth, then add bay leaves, salt, and pepper. Bring the creamy hearty lentil sausage soup to a boil, then reduce heat to medium-low and simmer for 25-30 minutes until lentils are completely tender.

Stir in the heavy cream until fully incorporated, heating through for 2-3 minutes without boiling. Add fresh spinach and let it wilt into the sausage soup for 1 minute, then remove bay leaves and taste for seasoning adjustments.