Preheat oven to 200°C
Line a baking tray with baking paper
Place the sliced onions on the tray and arrange the tomatoes, cut-side up, on top, tuck in the garlic cloves add the oil and season with salt, pepper, cumin, chilli flakes and a pinch of sugar
Bake for about 45 minutes until tomatoes softened and begin to char around the edges.
Add a small amount of oil to a large pot and add the basil stalks and cook for a minute
Fish out the garlic and add the tomatoes and onion plus any juices from the tray
Squeezed in the flesh from the garlic, discard the skins
Add the stock
Stir and bring to the boil, then turn the heat down, cover and leave to simmer for 25 minutes, leave to cool slightly
Use a hand held blender and purée the soup, then stir in the vinegar (if using), and season to taste. Reheat gently, divide in to bowls and tear the basil leaves into pieces and place on top of each serving.
Serve with crusty bread