Place the browned sausage, onion, carrots, celery, garlic, Italian seasoning, beef bouillon powder, salt, and broth in a 6-quart / litre slow cooker bowl. Cover and cook on high for 4 hours or low for 7 hours.
Uncover and skim any fat that is sitting on the top of the soup with a spoon; discard. Stir in the cornstarch mixture with the evaporated milk (or half and half or cream). Add the tortellini and mix well. Cover again and cook on HIGH heat setting for a further 45 minutes until the soup has thickened, and the tortellini is soft and cooked through.
Add in the spinach, pressing the leaves down to completely submerse into the liquid. Cover again for a further 5-10 minutes until the leaves have wilted.
Pour in milk in ⅓ cup increments, as needed, to reach your desired thickness and consistency (I needed 1 cup); taste test and season with extra salt ONLY if needed, and pepper to suit your tastes.
Serve with crusty warmed bread
Notes
*Italian sausage gives this soup an amazing flavour, but you can substitute it with ground chicken, turkey or beef sausage. OR use plain ground meats if you don't like sausage. For vegetarian options, leave the meat out all together.The soup thickens as it cools and absorbs quite a lot of liquid. Extra milk may be needed when reheating leftovers to reach your desired level of creaminess.