Heat the olive oil in a large saucepan over medium heat until shimmering. Add the diced onion and cook, stirring occasionally, until soft and translucent, about 5 minutes, and the kitchen fills with a sweet aroma.
Add the minced garlic to the saucepan and cook for another minute until fragrant, listening for a gentle sizzle and smelling the toasted aroma.
Stir in the chopped heirloom tomatoes and cook, stirring occasionally, for 10 minutes until the tomatoes are softened and their juices are released, turning the mixture a rich, bubbly red-orange.
Pour in the vegetable broth, bringing the mixture to a gentle boil. Reduce heat and simmer uncovered for 15 minutes, allowing the flavors to meld and the soup to thicken slightly.
Add the torn basil leaves directly into the soup, allowing their bright aroma to infuse as the mixture continues to simmer for 5 minutes.
Remove the soup from heat. Use an immersion blender inline or carefully transfer the mixture to a blender in batches to blend until smooth, creating a velvety, vibrant orange-red puree.
Return the pureed soup to the pot if using a traditional blender, then season with salt and freshly ground black pepper to taste. Warm gently over low heat for a few minutes.
Ladle the hot soup into bowls, garnish with additional torn basil or a drizzle of olive oil if desired, and serve immediately.
