March 18th, 2014
In a slow cooker, toss onions, olive oil, butter, brown sugar and salt, cover entire top of slow cooker with two layers of paper towel and secure with lid. This will catch any excess condensation in slow cooker and allow onions to turn golden brown. Cook on low until onions are soft and caramelized, about 8 hours. This can go longer to about 10 hours, if necessary.
Turn slow cooker to high, add beef stock, sherry and thyme. Cover and cook 30 to 45 minutes. Season to taste with salt and freshly ground black pepper.
Meanwhile, preheat broiler. Arrange bread slices on a sheet pan and lightly brush both sides with olive oil. Set under the broiler and broil, turning once, until lightly toasted on both sides, about 3 minutes.
Ladle soup into oven-safe bowls. Cover each bowl with a piece of toasted bread and cover with cheese. Place bowls under broiler until cheese is golden and bubbling, about 4 to 5 minutes.