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You can season and sautee the onions in a large skillet before throwing them into the slow cooker. It's up to you on how you want to do it.

If you decide to use the slow cooker then that is fine. I like my onions a little on the softer side that is why I prefer this method.

Here Are The Directions To Follow: 

1. Season the onions: Place the onion slices in a 5-quart (or larger) slow cooker. Stir in the butter and olive oil, salt, and a generous quantity of black pepper.
2. Cook on LOW for 12 hours: Cover and cook on LOW overnight or for 12 hours. The onions should be dark golden brown and soft.
3. Add broth: Stir in the balsamic vinegar and the broth.
4. Cook for 6 to 8 hours on LOW: Cover and continue cooking on LOW for 6 to 8 hours. This is flexible; as long as your slow cooker holds moisture well (wrap a towel over the lid if quite a lot of steam escapes), you can cook the soup for hours. Longer cooking will only intensify the flavors. Taste and season with more salt and pepper if desired, and stir in the brandy if using.
5. Portion the soup into oven-safe bowls: Heat the oven to 350°F. Ladle the soup and onions into oven-safe bowls and place the bowls in a baking pan.
6. Top with toast and shredded cheese: Top each bowl with a slice of toast and a generous quantity of shredded Gruyere cheese.
7. Bake for 20 to 30 minutes: Bake on a rack in the upper third of the oven until the cheese is completely melted.
8. Broil for 2 to 3 minutes

After the long wait is over, you can finally serve up your soup! Of course, though it took a full day to prepare, it'll probably be gone within an hour. Enjoy!

Article Source: The Kitchnn