French Onion Reuben Soup

Directions

• Place a large sauce/soup pot or Dutch oven over medium low heat. Add all onions to the pot. Dice the butter and place the cubes of butter on top of the onions. Stir occasionally. As the butter melts, continue stirring frequently until the onions are a rich, caramelized brown color. This can take up to an hour or more to properly caramelize. Do not allow the onions to burn. Scrape the bottom of the pot with a wooden spoon and continue cooking. Season the caramelized onions with salt and pepper.
• Add the sherry to deglaze the pan, scraping up any browned bits. Stir and cook until the sherry has reduced slightly and it no longer has a pungent, alcohol aroma. Add the stock, picked thyme, minced garlic, bay leaf, and the cooked corned beef. Turn the heat to medium and allow to gently simmer for about 20 minutes. 
• Add the Worcestershire Sauce and apple cider vinegar. Taste and season with salt and pepper as needed. Discard the bay leaf. 
• Preheat broiler to high or set your oven to 450. Adjust oven rack to the top position if using a broiler. Spread butter or oil over the thick cut rye bread, both sides. Place the bread on a sheet tray and place under the broiler. Toast each side but do not burn. 
• Set aside 4 oven proof soup bowls or crocks.
• Spoon a small amount of the broth into the bottoms of the 4 ovenproof bowls. Place a piece of toast on top of each. Sprinkle some of the grated cheese on top of the toast, then spoon more soup, onion, and corned beef on top, nearly filling the bowls. Top the soup with the 4 remaining rye toasts, pushing them down to nearly submerge them. Top with the remaining grated cheese. 
• Set the bowls on a foil-lined sheet tray and place the tray below the broiler or in the oven. Broil or bake until the cheese is melted and browned in spots.
• Garnish with fresh cut chives and serve.