Heat your oven to 200°C/Fan 180°C/Gas 6. Set a roasting tin on the top shelf to heat up.
Halve the aubergine lengthwise and cut into 1 cm-thick slices. Halve, peel and cut the onion into 2cm slices. Slice the pepper into 2cm-thick slices (discard the stem and seeds).
Tumble everything onto your preheated roasting tray. Add 1 tbsp oil. Season well. Shake so there’s just a single layer. Roast for 20 mins or till the veg is tender.
While the veg roast, peel and finely chop 1 garlic clove. Finely chop the oregano leaves. Mix with the zest from the lemon, 3 tbsp lemon juice and 3 tbsp oil. Blitz with a hand-held blender, if you have one, for a finer pesto. Season to taste.
Peel and chop the other garlic clove. Tip the tomatoes into the roasted veg with the garlic. Fill the tomato tin with hot water (around 400ml) and add to the veg. Return to the oven for 5 mins.
Take the veg out of the oven and blitz with a hand-held blender to make a smooth soup, or ladle everything into a blender and whizz till smooth. Add a little extra water, salt or pepper if you think it needs it. Serve the soup with a drizzle of the oregano pesto.