The secret ingredient in this soup is time. When you treat your onions with the love and care they deserve they will reward you with a rich, deep complex flavour. We recommend caramelising them for at least 45 minutes. That may seem long, but once you taste the incredible umami flavour you will not regret a moment of it.
We like to use a mix of red onions, white onions and shallots for maximum flavour, but feel free to use whatever is easiest for you. The amounts specified in the recipe come from about 13 small onions.