In a medium-sized soup pot, add onions, butter, salt and sugar on medium-low heat for about 15 minutes until onions are clear. The salt and sugar will help the onions sweat and break down.
Turn the heat down to low and add tomatoes and heavy cream. Stew on low heat for 45 minutes. The secret is allowing the flavors to develop slowly on low heat while stirring often.
After flavors have been given time to develop, add fresh basil and blend. Adjust seasoning as needed and enjoy.
A classic grilled cheese sandwich is the perfect pairing for this soup.