Slowly caramelize the sliced onions in a large pot with a little oil or butter over medium-low heat for about 30-45 minutes, stirring frequently, until deeply golden brown and sweet.

Add the sliced mushrooms to the caramelized onions and cook for another 5-10 minutes, until softened.

Deglaze the pot with the white wine, scraping up any browned bits from the bottom.

Pour in the beef broth and bring to a simmer. Add the Gravy Master and Better Than Bouillon (if using). Continue to simmer for at least 30 minutes to allow the flavors to meld.

Ladle the soup into oven-safe bowls. Top with shredded Gruyère cheese.

Serve immediately.