Add the crushed tomatoes, vegetable broth, diced onion, minced garlic, chopped basil, salt, and black pepper to the slow cooker.
Stir the ingredients until evenly combined.
Cover and cook on low for 6 hours.
Stir in the heavy cream and grated Parmesan cheese until the cheese melts into the soup.
Use an immersion blender to puree the soup until smooth.
Ladle into bowls and top with extra fresh basil and grated Parmesan before serving.