Preheat the oven to 140C or 300F. Place whole garlic bulb, Spanish onion, tomatoes and peppers, cut side up, on a baking tray lined with non-stick paper. Season the tomatoes and peppers with salt and pepper and drizzle with extra virgin olive oil.
Roast in oven for 30 minutes or until tomatoes are tender and the skin of the peppers is blistered and slightly blackened. Allow ingredients to cool for 15 minutes or so. Peel the skin of the peppers - they will just fall off.
Into the bowl of a food processor, squeeze the softened garlic. Add the tomatoes, peppers and onions. Process until smooth
Transfer the pureed vegetables into a large saucepan and heat over medium-high heat. Add the chicken stock and bring to the boil. Reduce heat to low. Simmer, stirring occasionally for 10 minutes or until the mixture thickens slightly. Taste and season with salt and pepper.
If you prefer a thicker soup, only add two cups or half a litre of stock. Remember this will now only serve 4.