This is one of the most fundamentally simple soups to make, but there are a few areas where you can go wrong. Having the patience and taking the time to properly caramelize the onions will result in a delicious soup without the need for any wine or sherry.
Over medium-low heat, melt the butter until it is foamy. Add the onions and a generous pinch of salt and stir to coat. Keeping the heat low, continue to cook the onions, stirring frequently, until they've broken down, caramelized and have turned a deep golden brown colour. This will take at least 45 minutes to one hour, if not more. Don't be tempted to increase the heat, they will caramelize, it will just take time.
In a large measuring cup, mix the apple cider vinegar with 60ml (1/4 cup) of chicken stock. Pour over the onions, scraping up any sticky browned bits of the bottom of the pan. Allow to simmer until mostly evaporated.
Increase the heat to medium-high and pour over remaining chicken stock. Bring to a boil, reduce the heat to a simmer, cover and allow to simmer for 20-30 minutes.
Top slices of bread with grated cheese and transfer to the broiler until the cheese is melted and lightly browned. Cut the toasts in half. Ladle a small amount of soup into bowls, put one half of your cheese toasts on top of the soup, and fill the bowl, topping with the other half. Serve immediately.
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Notes
Homemade chicken stock is your best bet, but if you don't have any, use a high-quality storebought low-sodium chicken stock instead. This recipe is also very good with homemade beef stock, but do not use storebought beef stock as there are commonly a number of additives that can lend a funky flavour to the finished product.