Dice the vegetables finely so they cook evenly.
Only puree the soup when the vegetables are really soft.
The binding in the soup comes from the vegetables. If the soup is too thick after pureeing, stir in some water, stock or cream until you have the desired consistency.
Only add the fresh basil when serving the soup. If not, the fresh basil will turn brown quickly.
You can enrich the soup by using fresh tomatoes rather than tinned ones. Cut them in half, sprinkle with some salt and pepper and roast in the oven until soft and lovely.
Add some fennel to the soup base.