With puff pastry.
SOUP: In thick cast iron pot (with a lid), melt butter over medium heat.
Add onions and salt, cover and cook until softened, stirring occasionally about 15 minutes.
Uncover and cook until caramelized, about 30 minutes more.
Stir in flour; cook 2 minutes. Stir in remaining ingredients. Simmer, partially covered about 30 minutes. Remove bay leaf. Cool and refrigerate.
PASTRY PUFF: Preheat the oven to 400°F Roll out one sheet of puff pastry and sprinkle with half the Gruyere cheese, fold into 3 and roll out again. Cut out 4 lids. Divide the soup among ovenproof soup bowls. Sprinkle the remaining cheese on top of soup and cover with the pastry lids with a little egg wash seal. Bake the soup in the oven for 15 minutes or until the cheesy puff pastry is golden in colour and well risen.