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. Mix cornstarch and water until smooth. 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 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 ½ cup); taste test and season with extra salt and pepper to suit your tastes.