Add the butter and olive oil to the bottom of your slow-cooker. Add the thyme, bay leaf, and chopped garlic. Add the onions. Sprinkle with sea salt and fresh pepper. Cover and cook on high for 8 hours.
In a saucepan, heat the beef stock and brandy to a boil. Add to the cooked onions and cook on high in the slow-cooker for about 10 minutes. Remove the bay leaf.
To serve the soup, ladle into oven-safe bowls over a piece of baguette. Sprinkle with 2 tablespoons cheese. Place bowls on a sheet pan under the broiler until the cheese is bubbly.