Preheat your oven to 400°F (200°C). Place the halved tomatoes and peeled garlic cloves on a baking sheet. Drizzle with olive oil, ensuring each tomato is coated. Sprinkle with salt and black pepper. Roast in the oven for 40-45 minutes or until the tomatoes are caramelized and fragrant.

While the tomatoes are roasting, heat olive oil in a large pot over medium heat. Add thinly sliced onions and cook until golden brown and caramelized. This process adds a sweet depth to the soup, complementing the acidity of the tomatoes.

Once you have roasted the tomatoes and garlic, let them cool down for about 10 minutes. After that, transfer them to a food processor or blender and blend until they become smooth. Following this, add the caramelized onions and basil and blend once again. Alternatively, you can use an immersion blender to puree the soup directly in the pot.

Transfer the mixture into the pan and add water or vegetarian broth according to desired consistency, using less for a thicker soup and more for a lighter version.

Allow the soup to simmer over low heat for 15-20 minutes, letting all the flavors meld together. The kitchen will be filled with an irresistible aroma, building anticipation for the deliciousness that awaits.