Melt the duck fat in a pot and sauté the chopped garlic.

Pour in the mussel juice and enough water to make a total of 6 1/3 cups (about 1.5 liters). If you prefer it less salty, use less mussel juice. Heat the mixture.

Separate the egg whites from the yolks.

Add the egg whites to the hot broth and whisk vigorously over high heat to create thin strands.

In a separate bowl, mix the egg yolks with the balsamic vinegar.

Add this mixture to the broth and let it cook until the soup turns pale.

Add the garlic croutons just before serving.