Preheat the oven to 400°F and line a baking sheet with parchment paper, set aside.
Add tomatoes and garlic to a large bowl and toss them with 3 Tablespoons of olive oil. Spread tomatoes and garlic onto the prepared baking sheet in a single layer and sprinkle with ½ tsp salt and ½ tsp pepper. Roast for 40 minutes.
While tomatoes are roasting, in a Dutch oven or large pot, heat remaining 1 Tablespoon of olive oil over medium heat. Add the chopped onions, ½ tsp of salt and ¼ tsp pepper. Sauté, stirring occasionally until onions are translucent and start to brown and caramelize, about 20 minutes.
Once onions are caramelized add Italian seasoning and vegetable broth. Let simmer for 5-10 minutes.
At this point your tomatoes should be done roasting. Remove from the oven and let them cool slightly before transferring the tomatoes, along with the garlic and juices into the Dutch oven.
Turn down heat, add fresh basil and use an immersion blender to blend the soup until smooth. If you don’t have an immersion blender you can use a blender. Carefully transfer the soup mixture into your blender and blend until smooth or until you reach your desired consistency. You may need to do this in batches, depending on the size of your blender.
Taste and add any additional salt and pepper and more vegetable broth, if desired, to reach your preferred consistency.
When ready to serve, divide soup into 4 bowls. Garnish with fresh basil and serve with parmesan cheese, as well as, crusty bread, grilled cheese or crackers.