If using sausage, brown it in a skillet over medium-high heat until cooked through and crumbled. Drain excess grease.
Add onion, carrots, celery, garlic, broth, diced tomatoes, tomato paste, Italian seasoning, salt, pepper, and red pepper flakes (if using) to the slow cooker. Stir until the tomato paste dissolves.
Add cooked sausage (if using).
Cover and cook on LOW for 5 to 6 hours or HIGH for 3 to 4 hours, until vegetables are tender.
Add cubed cream cheese. Cover for 10 to 15 minutes to soften, then whisk or stir until smooth. Stir in heavy cream and Parmesan.
Add tortellini. Cover and cook until tender (20 to 30 minutes for refrigerated; 30 to 45 minutes for frozen).
Stir in spinach and cook 3 to 5 minutes, just until wilted. Taste and adjust seasoning. Serve with extra Parmesan and black pepper.