Melt the butter in a large, heavy-based saucepan over a medium heat. Add the onions and a little seasoning and sweat for over a reduced heat for 25-30 minutes, stirring regularly, until very soft and starting to colour.
Increase the heat a little and continue to cook, stirring regularly, until dark brown in colour and nicely caramelized. Deglaze the saucepan with the brandy until almost entirely evaporated.
Add the bouquet garni and beef stock and bring to the boil, then reduce to a simmer for 15-20 minutes. Remove from the heat and adjust the seasoning as necessary.
Pre-heat the grill to hot. Spoon the soup into heatproof soup bowls and top the surface evenly with the baguette slices. Sprinkle the Gruyère evenly on top and gratinee under the grill until toasted and golden brown in colour. Remove and allow to stand for a few minutes before serving.
Freezing and defrosting guidelines
Make the soup, then leave to cool at room temperature. Freeze (without garnishes or toppings) in a rigid container, leaving a bit of space for expansion, for up to 1-3 months. Reheat either from frozen or defrost in the fridge overnight. Once piping hot, add toppings or garnishes and serve.