Preheat the oven to 400 F. Cut the tomatoes and onion into 1/2" wedges and transfer to an oven safe pot. Add the garlic, olive oil, salt, and pepper and toss until well coated.
Transfer the pot to the oven and cook, uncovered, for 30 minutes.
If you have an immersion blender, add the vegetable broth, milk, and fresh basil to the pot and use the immersion blender to blend until smooth. If you don't have an immersion blender, carefully transfer the cooked vegetables to a blender, add the vegetable broth, milk, and basil, and blend until smooth.
Scoop the soup into bowls and serve while it is hot!