Add tomato paste to the slow cooker first. Then add crushed tomatoes, fire-roasted diced tomatoes, broth, garlic, onion, Italian seasoning, basil, oregano, red pepper flakes, salt, and pepper. Drop in the parmesan rind. Stir to combine.
Add pre-browned Italian sausage or raw chicken breast on top of the tomato base. Skip for vegetarian.
Cover and cook on LOW for 4 to 5 hours, or HIGH for 2 to 3 hours. Shred chicken if used. Remove and discard parmesan rind.
Use an immersion blender for 8 to 10 pulses to partially smooth the base. Taste and adjust acidity with sugar or balsamic vinegar if needed.
Switch to HIGH. Add refrigerated tortellini for final 20 to 25 minutes, or frozen for final 28 to 35 minutes. Test doneness before moving on.
Switch to WARM. Ladle half a cup of hot soup into a small bowl with room-temperature heavy cream and whisk to temper. Pour tempered cream back into slow cooker slowly, stirring gently. Do not boil after adding cream.
Stir in baby spinach and wilt for 2 minutes. Season to taste. Serve hot with fresh basil, parmesan, and black pepper.