Cook sausage in a large Dutch oven or soup pot over medium-high about 8 minutes, breaking up with a wooden spoon, until browned and cooked through; drain excess fat if needed.
Cook onion, carrots, and celery with sausage 5-7 minutes, stirring occasionally, until starting to soften.
Add garlic, oregano, basil, and optional pepper flakes; cook 1 minute until fragrant.
Stir in lentils, tomatoes with juice, and broth.
Boil; reduce heat to low, cover, and simmer 30-35 minutes until lentils are tender, stirring occasionally.
Add spinach; cook 2-3 minutes until wilted and season with salt and pepper.
Ladle into bowls; top with fresh Parmesan and serve with crusty Italian bread.