Line a large baking sheet with parchment paper. Place halved tomatoes and garlic cloves on the baking sheet and drizzle with 3 tablespoons of olive oil. Generously season with salt and pepper. Roast in the oven for 40-45 minutes.
Add 1/2 tablespoon olive oil to a large pot and place over medium heat. Add the onion slices and stir to coat the onions with olive oil. Cook, stirring occasionally. Check onions every 5-10 minutes until they have completely caramelized and turned golden in color. This ususally takes 20 minutes.
Once tomatoes and garlic are done roasted, allow them to cool for 10 minutes, then add them to a food processor or high powered blender and blend until smooth. Next add basil and caramelized onions and blend again.
After blending, transfer back to pot, turn to medium low heat and add in oregano, vegetarian broth and salt and pepper to taste. From there you can add in any additional add-ons you want (I.e. Whole dairy milk/heavy cream for a creamy texture. Parmesan cheese, for a tangy, flavor enhancing flavor, a tablespoon or two of butter, for richer flavor) Allow tomato soup to simmer 10 minutes before serving. To serve, garnish with parmesan cheese and serve with grilled cheese, if desired.