Remove the casings from the Italian Sausage, crumble, and brown over medium-high heat.
Add the sausage, onion, celery, garlic, seasonings, and beef stock to the crockpot. Cook on low heat for 6 hours.
Whisk the corn starch and water until well combined. Add to the crockpot along with the milk and spinach. Stir, turn heat to high and cook until the soup begins to thicken. This will take about an hour.
Meanwhile, cook the tortellini according to package directions. Drain but do not rinse.
Just prior to serving, add the tortellini to the crockpot and heat through.