Remove casings from sausage. Cook sausage, onion, and garlic in a large Dutch oven over medium heat until sausage is browned, stirring to crumble. Drain well; return to pan.
Add tomatoes and next 3 ingredients; bring to a boil. Cover; reduce heat, and simmer 15 minutes. Add tortellini; cover and simmer 6 minutes, stirring occasionally. Stir in spinach; cook 2 minutes.