Melt butter in a large saucepan over low heat, add onion and season to taste with sea salt. Cook slowly, stirring often, until onion begins to wilt and release liquid (20-25 minutes). Increase heat to medium-high, stir occasionally until liquid reduces completely, then stir often to ensure it colours evenly and slowly to fully develop the flavour, until deeply caramelised (25-30 minutes). Add Sherry vinegar and Madeira and reduce until almost evaporated (1-2 minutes). Add stocks, bring to the boil, reduce heat to medium, cover and simmer until well-flavoured (1-1½ hours). Season to taste, adding a few more drops of vinegar if necessary.
Preheat grill to high heat. Place baguette slices on an oven tray and toast, turning once, until golden (1-2 minutes each side).
Divide onion soup among four deep ovenproof serving bowls and top each with a toasted croûton. Combine cheeses in a separate bowl, scatter evenly over soup and place bowls on an oven tray. Grill (see note) until golden and bubbling and serve hot, scattered with parsley.
Note If you can't fit soup bowls under your grill, preheat
oven to 220C and bake soup until golden and bubbling (4-5
This recipe is from the July 2012 issue of Australian Gourmet