In a large pot over medium heat, cook the sausage until browned, being sure to crumble it as it cooks. Transfer sausage from the pot to a plate or bowl with a slotted spoon. You may want to line the plate or bowl with a paper towel if your sausage is excessively greasy.
Using the leftover grease from the sausage, cook the onion and garlic until onion is tender, about 5-7 minutes. Gradually add the chicken stock and allow mixture to come to a low simmer.
Add back in the sausage, spinach, and heavy cream or milk, and bring to a simmer again. Add in the tortellini and cook according to the amount of time on the tortellini package (likely 3-5 minutes).
Remove from heat, taste, and add salt/pepper as needed. Allow to cool 15 minutes before serving. Store leftovers covered in the refrigerator up to 5 days. Soup freezes well, up to 2 months. Thaw in refrigerator overnight.