Cook sausage and onions in a 6 quart Dutch oven or soup pot over medium high heat, crumbling sausage with a wooden spoon while it is cooking. Cook until sausage is browned, no longer pink and fully cooked.
Add garlic and cook until fragrant (about 30 seconds), stirring constantly.
Drain off excess fat.
Add chicken broth, diced tomatoes with juices, Italian seasoning, black pepper and bay leaf. Stir to combine and bring to a boil.
Add tortellini and return to a boil. Cook for 5 to 8 minutes, or until tortellini is almost tender, stirring occasionally.
Reduce heat to medium low and add spinach. Stir to combine. Cook an additional 2 minutes, or until spinach is wilted and tortellini is tender.
Remove and discard bay leaf.
Sprinkle with Parmesan cheese when serving, if desired.