Start by peeling your onions. Slice them into thin, even half-moons. Consistency is key here so they all cook at the same rate. Don't worry if it looks like too many onions. They will shrink down.

Melt the butter and olive oil in your large pot over medium heat. Add the sliced onions and stir to coat them well. Let them cook slowly. This is the foundation of your amazing soup.

Sprinkle the sugar over the onions. This helps them turn a beautiful golden brown. Cook for about 30 to 40 minutes. Stir often so they do not burn or stick to the bottom.

Once the onions are dark and soft, add the minced garlic. Cook for just one minute until fragrant. Pour in the sherry or brandy to scrape up the tasty brown bits from the pot.

Pour in the beef stock, thyme, and the bay leaf. Bring the liquid to a boil. Then, lower the heat and let it simmer for about 20 minutes. This blends all the flavors together.

Remove the thyme sprigs and the bay leaf. Stir in the balsamic vinegar for a bit of tang. Add salt and pepper. Taste it to make sure it is exactly how you like it.

Preheat your oven or broiler. Place the baguette slices on a baking sheet. Toast them until they are golden and crunchy. This prevents the bread from getting too soggy in the soup later.

Ladle the hot soup into oven-safe bowls. Top each bowl with a toasted bread slice. Pile on the Gruyère and Parmesan cheese. Place the bowls on a baking sheet for safety.

Place the bowls under the broiler for 2 to 3 minutes. Watch them closely so the cheese doesn't burn. Remove them once the cheese is melted, brown, and bubbling over the sides.