In a large pot, heat the butter over medium heat until melted.
Add the chopped onion and give it a good stir. Put the lid on and let them sweat for about 5 minutes.
Uncover the pot, stir in the garlic until fragrant, about one minute.
Add the drained beans, crushed tomatoes, brown sugar, balsamic vinegar and salt. Stir well to combine and bring the soup to a simmer. Cover the pot with a lid and allow the soup to simmer for 15 minutes.
Take the soup off the heat. Add the basil. Use an immersion blender right in the pot to blend until smooth. Taste the soup and adjust seasonings if necessary. Return the soup pot to the heat and keep warm until ready to serve.