In a large pot, heat the butter and olive oil over medium heat.
Add the diced onion and minced garlic.
Cook until they are soft and translucent, about 5 minutes.
Pour in the canned tomatoes and chicken broth.
Stir well and bring to a simmer.
Let the mixture cook for about 15 minutes so the flavors meld together.
Use an immersion blender to puree the soup until smooth.
Alternatively, transfer it in batches to a countertop blender.
Stir in the sugar, heavy cream, and sherry (if using).
Season with salt and pepper to taste.
Continue to simmer for another 10 minutes.
Serve the soup warm with your choice of croutons or a side of bread.