Chop the tomatoes and onion into large, even piece (quarters is fine). Place into a large bowl.
Separate the garlic cloves but leave them unpeeled and place into the bowl.
Drizzle over one tablespoon of vegetable oil and toss the vegetables to coat well.
Spread the vegetables out onto a large baking tray, sprinkle with plenty of fresh thyme and season with salt and pepper to taste.
Roast for one hour, checking occasionally and turning the vegetables so they do not blacken.
Remove the baking tray from the oven and leave the vegetables to cool.
When cooled, the garlic will slip out of its skin. Place the roasted garlic (without the skin) and the rest of the vegetables into a blender along with the hot water and vegetable stock (if using). Blend until it reaches the desired consistency, adding more hot water if you need.
Leave the soup as is if you don't mind tomato seeds and skin in it, alternatively, strain for a smoother soup.