Sprinkle sugar over onions once wilted to encourage caramelization process- you don’t need to do this step, I find it speeds the process a bit.
Cook onions until they are golden brown and caramelized.
Add salt, pepper, red wine, and beef stock and simmer.
Add additional beef broth to soup and cook a low until combined. I let mine sit on top of the stove for an hour or so on low to let the flavors mellow.
Serve into individual servings in oven safe dishes, top with croutons or bread, and top with Gruyere cheese and bake until bubbly (I sprinkle shredded Gruyere on mine and call it a day).