Heat your oven to 200°C/Fan 180°C/Gas 6. Trim the green top off the aubergine, then roughly chop. Tip into a roasting tin. Peel and crush or grate the garlic. Add to the aubergine. Strip the leaves off most of the rosemary sprigs, saving 1 for later. Roughly chop the leaves and add to the tin.
Add 1 tbsp oil and some salt and pepper. Mix everything with your hands to coat the aubergine in the oil, garlic, rosemary and seasoning. Roast for 30 mins till the aubergine is tender and a little charred.
When the aubergine is roasted, tip it into a blender and add the chopped tomatoes and a tin of water (around 400ml). Blitz to make a smooth soup. If you don’t have a blender, tip the aubergine into a pan and add the tomatoes. You can blend it with a hand-held blender or leave it chunky. Warm through before serving.