Preheat the oven to 200 degrees celcius or 400 farenheit
Coat the tomato, garlic and onion with olive oil, put them cut side down on a baking dish and bake for about 25 minutes.
If roasting cherry tomatoes, put them aside when done to add as topping on the soup.
After this remove the tomatoes from the oven and add to a blender or pan (if using an immersion blender) with the basil and spices (and maybe a hot pepper) and blend with the water or vegetable stock until smooth.
Now you can bring it to a boil to simmer for about 10 minutes while adding salt and pepper to taste.
Enjoy!