Preheat oven to 400 degrees.
Place tomatoes on a large parchment-paper-lined baking pan in a single layer.
Add onions and garlic on the same baking pan, and drizzle olive oil, sprinkle 1 teaspoon salt and ½ teaspoon pepper over tomatoes and onions.
Bake in preheated oven for 40-45 minutes.
Combine canned tomatoes, broth, basil, thyme, and milk in a Vitamix or in the bowl of a food processor. Process for 1-2 minutes, or until completely smooth. (Process in two batches if using the processor.) Pour processed tomatoes into a large pot or Dutch oven.
Add the roasted tomatoes, onions, and garlic to the Vitamix or processor along with any oil that remains on the parchment paper. Process until smooth and combine the vegetables with the ingredients already in the pot.
Additionally, you can add the tomatoes, broth, basil, thyme, milk, and everything that was roasted in the oven to a large pot or Dutch oven and then puree using an immersion blender.
Bring the soup to a boil and then reduce heat to medium-low. Cover and simmer for 10-15 minutes.
Stir in remaining 1 ¼ teaspoon salt and ¼ teaspoon pepper to taste.
Serve roasted tomato basil soup with a toasted baguette, homemade croutons, or grilled cheese sandwich and enjoy!