In a large, dry frying pan, sauté the onions on a medium heat until browned. Add the butter and continue to cook until they become a deep brown colour. Add the garlic and continue to stir for 3 minutes. Transfer the caramelised onions to a large heavy-based pan with the wine, hot stock, brandy and balsamic - heat on medium and bring to a simmer. Leave to simmer for 15 minutes, uncovered.
Rinse and wipe the frying pan clean - now you can use it for the toasties!
Butter one side of each slice of bread. Place the Gruyère cheese slices on the non-buttered side of one of the slices. Make a sandwich by placing the other non-buttered side on-top of the cheese, leaving the two buttered sides of the bread facing out.
Fry the Gruyère sandwich for about 5 minutes over a medium-high heat in the frying pan, until the underside is golden. Flip and fry the other side. If the cheese hasn't melted, turn the heat down and continue to fry until the cheese starts to ooze out of the sandwich (being careful not to burn the bread).
Cut the crusts off of the Gruyère toastie then cut it diagonally into quarters.
Remove the soup from the heat, season to taste and ladle into heatproof bowls. Serve the Gruyère toasties warm with the soup.