All you really need is a blender and a fridge. It's very beautiful after a day or so when things like raw onion have calmed down a bit (less of a problem with the sweet white onions they have here. Use spring onions rather than the brown ones we have at home).
Method
Soak a piece of white bread in a little bit of veg stock and when it's soft, add it to a blender with the rest of the stock, the onion, garlic...everything except the tomato pasata - and blend it to a puree.
Add the pasata, olive oil and sherry vinegar and a bit of salt and pepper and mix it together.
Put it in the fridge and eat it the next day
It's usually garnished with chopped peppers, onions, cucumber and tomatoes, it's up to you