Step 1: Heat the olive oil in a large Dutch oven or heavy-bottomed pot over medium heat. Add the finely diced sweet onion and sauté for 8-10 minutes, or until softened and translucent, stirring occasionally. This slow cooking caramelizes the onions and builds a foundational sweetness for your creamy tomato soup.
Step 2: Add the minced garlic and optional red pepper flakes to the pot. Sauté for another 1 minute until fragrant, being careful not to burn the garlic.
Step 3: Pour in the crushed San Marzano tomatoes and diced tomatoes (with their juices). Add the vegetable broth, dried basil, and dried oregano. Stir everything together. Bring the mixture to a gentle simmer.
Step 4: Reduce the heat to low, cover the pot, and let the creamy tomato soup simmer for at least 20 minutes. This allows the flavors to meld and deepen. For a richer flavor, you can simmer it for up to 30-40 minutes.
Step 5: Carefully remove the soup from the heat. For a smooth consistency, use an immersion blender to purée the soup directly in the pot until silky smooth. Alternatively, transfer the soup in batches to a standard blender, being very cautious with hot liquids (vent the lid and cover with a towel).
Step 6: Return the puréed soup to the pot over low heat. Stir in the heavy cream (or coconut milk) and the optional unsalted butter. Whisk gently until fully combined and heated through. Do not boil after adding the cream, just warm it. Season the creamy tomato soup generously with salt and freshly ground black pepper to taste.
Step 7: Ladle the warm creamy tomato soup into bowls. Garnish with fresh basil leaves, and serve immediately with croutons, a drizzle of olive oil, or a classic grilled cheese sandwich. Enjoy your homemade creamy tomato soup!