In a heavy skillet heat the oil over medium-high heat. Add onion and cook, stirring, until tender, about 4 minutes. Add the garlic and cook until fragrant, 30 second. Transfer to a 2-quart slow cooker.
Add the Italian sausage to the skillet and cook until lightly browned on both sides, about 4 minutes and transfer to the slow cooker.
Stir in diced tomatoes, chicken broth, water, and fennel. Cover and cook low for 6 hours.
After 6 hours, uncover and stir in the spinach, tortellini, and heavy cream. Increase to high setting, cover and continue cooking for an additional 30 minutes or until the tortellini is tender.
Top the soup with grated parmesan when serving.