Sweat the onion, garlic and thyme in the butter. Fry the tomatoes and bananas for about 3 minutes. Pour in the bouillon and add the bay leaf, bring to the boil. Reduce the heat and cover the soup and let it simmer for about 25 minutes. Remove the bay leaf and finely puree the bananas in the liquid. Add the peppers and cover and let the soup simmer for another 5 minutes.