In a large Dutch oven or soup pot, cook the Italian sausage over medium-high heat, breaking it up with a wooden spoon, until browned and cooked through, about 8 minutes. Drain excess fat if necessary.
Add the diced onion, carrots, and celery to the pot with the sausage. Cook for 5-7 minutes, stirring occasionally, until the vegetables begin to soften.
Stir in the minced garlic, oregano, basil, and red pepper flakes (if using). Cook for 1 minute until fragrant.
Add the lentils, diced tomatoes with their juice, and chicken broth. Stir well to combine.
Bring the soup to a boil, then reduce heat to low. Cover and simmer for 30-35 minutes, or until the lentils are tender, stirring occasionally.
Stir in the fresh spinach and cook for 2-3 minutes until wilted. Season with salt and pepper to taste.
Ladle into bowls and top with freshly grated Parmesan cheese. Serve with crusty Italian bread for the full Carrabba's experience!