Add the crushed tomatoes, vegetable broth, chopped onion, minced garlic, chopped basil, oregano, salt, and pepper to the slow cooker. Stir to combine.
Cover and cook on low for 6 hours or on high for 3 hours until the onion is soft.
Use an immersion blender to puree the soup until smooth. If you do not have an immersion blender, carefully transfer the soup in batches to a countertop blender.
Stir in the heavy cream until fully mixed. Let the soup warm for 10 more minutes on low.
Ladle into bowls and top with extra fresh basil leaves if desired.