Sauté the onion with a pinch of salt large pot for 5 minutes, until soft and translucent. Add the garlic and thyme and continue to sauté a couple minutes more.Add the canned tomatoes, broth and salt. Bring to a boil, then immediately turn the heat to a simmer. Simmer with the lid off for 30 minutes, then stir in the fresh basil. Use an immersion blender to purée the soup to a smooth consistency, or transfer the soup to a food processor to purée, then pour it back into the pot once smooth. Stir in the balsamic vinegar and heavy cream and serve.