Roast the tomatoes. Preheat the oven to 425 and prepare a baking sheet by lining it with aluminum foil. Place tomatoes on the baking sheet, sprinkle the brown sugar on top, and roast for 30 minutes, until they look shriveled and the edges are starting to brown. When they are done, allow them to cool slightly and then remove from the foil and set aside.
Cook the onion. Heat the butter in a large saucepan over medium heat until it foams, then add the onion and cook until softened, about 5 minutes. Add the tomato paste and stir to combine.
Add liquid and blend. Pour in the chicken broth and water slowly, whisking to combine. Add the roasted tomatoes back into the soup. Bring to a boil, then reduce heat and simmer, covered, for 10 minutes. Puree the soup right in the pot using an immersion blender, or puree in batches in the food processor (be careful not to splash yourself!)
Add cream and seasoning. Add the cream, sherry, salt, pepper, nutmeg, and cayenne (if using), and heat for 15 minutes more. Serve in bowls, topped with fresh basil.