For the soup, heat the oil in a pot over medium heat. Add the onions, cover with a lid and cook, 20 minutes. Stir regularly. Stir in the cake flour. Whisk in the Coca-Cola, stock, garlic and thyme. Simmer for 20 minutes and season to taste.
Discard the thyme sprigs and remove 500ml (2 cups) of the onions from the soup and strain it through a sieve. Keep the liquid being strained off and pour it back into the soup. Keep the soup warm over low heat.
For the jaffles, butter the bread slices on one side. Turn them over so that the butter is on the outside of each sandwich being made. Divide the onions and Gruyere cheese between four of the slices. Top each off with the remaining buttered bread slices. Heat the jaffle pan over medium heat until hot. Grease it with some non-stick cooking spray. Cook each sandwich in the jaffle pan for 2-3 minutes on each side until golden and crisp.
Divide the remaining soup between 4 serving bowls and serve each with a jaffle for dipping. Serve with additional Gruyere cheese, if preferred.
If you do not own a jaffle pan then simply make regular toasted sandwiches from the onions in an electric sandwich maker or fry the sandwiches off in a skillet with some butter until golden and crisp.
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