Preheat a large pan or stockpot over medium heat. Add the olive oil to the pot. Cook the hot or mild Italian sausage and chopped onion for 6- minutes until cooked through.
Add the minced garlic and Italian seasoning and cook for one more minute.
Drain most of the grease from the pan, I like to keep at least 1-2 Tbsp in the pot to go into the soup since it adds a lot of flavor.
Put the seasoned sausage mixture into a large crockpot, be sure to scape up the bits of sausage from the bottom of the pan you cooked it in and add those to the crockpot also.
Add the chicken broth and can of diced tomatoes. Stir all ingredients together.
Cook the tortellini soup crock pot recipe for 7-8 hours on low or 4-5 hours on high.
30 minutes before the cooking time is done, add the tortellini, spinach, cream, and shredded parmesan to the pot. Turn the slow cooker to High and cook until the tortellini floats to the surface and the soup is heated throughout. Give the soup a stir before serving.
Garnish with red pepper flakes, more parmesan, and/or fresh basil. Season with salt and pepper as desired.