Heat oil in skillet on medium heat. Add onions and garlic; cook and stir until tender crisp. Transfer to slow cooker pot.
Stir in remaining ingredients, except spinach and tortellini.
Cover and cook 4 to 5 hours on HIGH or 8-9 hours on LOW.
Stir in spinach and tortellini. Cook an additional 10 min. or until heated through
Top with shredded Parmesan cheese, if desired.
Serve with a crisp green salad and a slice of crusty bread