Add the whole peeled tomatoes with their juice, quartered onion, garlic cloves, vegetable broth, sugar, dried basil, salt, and pepper to the slow cooker.
Cover and cook on low for 6-8 hours or on high for 3-4 hours until vegetables are very tender.
Use an immersion blender to puree the soup until completely smooth, or carefully transfer to a regular blender in batches.
Stir in the heavy cream and butter until well combined and butter is melted.
Taste and adjust seasonings as needed, then serve hot with crusty bread or grilled cheese sandwiches.