Combine the diced onion, minced garlic, crushed tomatoes, tomato paste, broth, dried basil, dried oregano, salt, black pepper, and sugar in the Crock-Pot. Stir well to mix everything together.
Cover the Crock-Pot and cook on low heat until the vegetables are very soft, about 4 hours.
Blend the soup with an immersion blender until smooth. Then, stir in the butter and heavy cream, allowing them to melt and mix thoroughly. Taste and adjust seasoning as necessary.