Preheat oven to high grill.
Thinly slice onion and mince garlic. Set aside separately. Heat butter in a large pot on low-medium heat. Add onion and cook for about 25 minutes, stirring occasionally, until onions are extremely soft and caramelised. Reduce heat if onion starts to burn. Add garlic, balsamic vinegar and sugar and cook a further 5 minutes.
Add gravy mix, whisk to combine and cook for 1 additional minute. Add water measure and soy sauce to pot and cook a further 2 minutes, scraping the bottom the pot to release any brownings, until hot. Season to taste with salt and pepper.
Ladle soup into 4 ramekins. Slice baguette into 2cm slices and place slices on top of each ramekin. Sprinkle Parmesan over bread. Transfer ramekins to an oven tray.
Grill soups for 3-5 minutes, until golden. Keep an eye on them so they don't burn. Allow soup to cool down for at least 5 minutes before eating.