Begin by preparing all the vegetables. Chop the onion, dice the carrots and celery, and mince the garlic.
In the slow cooker, combine the vegetable broth, diced tomatoes (with juices), chopped onion, minced garlic, diced carrots, diced celery, dried basil, oregano, salt, and black pepper.
Stir the mixture gently to combine all ingredients.
Cover the slow cooker and set it on low for 4 hours.
About 30 minutes before serving, add the tortellini to the slow cooker and stir to combine.
If using, stir in the heavy cream for a richer flavor just before serving.
Taste the soup and adjust seasoning if necessary.
Serve hot, garnished with fresh basil leaves if desired.