Heat your oven to 200°C/Fan 180°C/Gas 6. Roughly chop the tomatoes and put in a roasting tin. Drizzle over ½ tbsp oil. Season. Drop in the unpeeled garlic clove. Roast for 30 mins till the tomatoes are soft and charred.
Meanwhile, peel and chop the onion. Peel and grate the ginger. Finely chop the coriander stalks, setting the leaves aside.
Heat ½ tbsp oil in a large pan over a low heat. Add the onion and season. Fry for 5-6 mins till golden and soft. Stir now and then. Add the ginger and coriander stalks. Stir. Pour in the coconut milk and 700ml boiling water. Pop on a lid. Simmer for 5 mins, then take off the heat.
When the tomatoes have finished roasting, pluck the garlic clove from the tin. Squeeze the flesh into the pan. Tip in the roast tomatoes. Put the pan back on the heat. Simmer for 5 mins.
Ladle the soup into a blender and blitz till smooth, or use a hand-held blender in the pan. Squeeze in a little lime juice. Taste and add more seasoning or lime juice if it needs it. Stir in most of the coriander leaves. Ladle into bowls. Serve garnished with the rest of the coriander.