To cook the onions, sauté the onions over medium heat for 20-25 minutes until they are soft and golden brown. Be patient as this is where the onions develop their rich flavour.
When the onions are golden brown, add the sugar, Balsamic vinegar and salt and cook for another 10 minutes over low-medium heat.
Remove from the heat and allow to cool completely.
Pre-heat the oven to 180°c and line two baking sheets with baking paper.
Roll out the puff pastry slightly then cut out rounds of approximately 3-5cm in diameter. I got around 15 rounds out of each sheet.
Brush each pastry round with egg wash. Top each round with a spoonful of the caramelised onions followed by some grated Gruyère. Take care not to overload each round with filling as it can burst open when baking.
Cover each onion and cheese-topped round with another round of pastry then crimp the sides with a fork.
Place the puffs on the baking sheet and brush with the egg wash. Place in the oven and allow to bake for 12-15 minutes until golden brown and puffed.
Remove from the oven and allow to cool slightly then serve.
French onion soup puffs https://simply-delicious-food.com/french-onion-soup-puffs/