5tablespoonsunsalted butter, divided, see Recipe Notes
3lbsVidalia onions, about 4 medium, halved lengthwise, peeled, and thinly sliced
1½cupsdry white wine
4cupsbeef stock, or store-bought low-sodium beef broth
1garlic clove, cut in half lengthwise
2teaspoonssherry, preferably Fino or Manzanilla, see Recipe Notes
1cupGruyère or Emmentaler cheese, grated
Thinly slice the onions using a mandolin slicer. Be sure to use the food guard to protect your fingers.
Melt 3 tablespoons butter over medium heat in an enameled cast-iron pot (or another heavy pot with a lid). Add the onions, sprinkle with 2 teaspoons of salt, then cover and cook until the onions have heated through and started to steam.
Uncover the pot, reduce the heat to low, and cook, stirring occasionally. Season with several grinds of pepper. Allow the onions to slowly caramelize, about 3 hours.
When the onions have completely cooked down, the water has cooked off, and the onions have turned amber, add the wine and raise heat to high. Cook until almost all liquid has evaporated, about 8 to 10 minutes.
Tie the thyme and bay leaves into a bundle with twine. Add the stock, water, and herb bundle to the pot with the onions. Bring to a boil, reduce to a simmer, and cook, uncovered, until the soup is thickened and flavorful, 20 to 30 minutes.
Remove the pot from heat and whisk in the remaining 2 tablespoons of butter. Taste and adjust the seasoning as needed. If desired, you can add ½ teaspoons sugar if the soup isn’t sweet enough.
Heat the broiler or have a toaster ready. Cut two ½-inch baguette slices for every serving of soup. Toast the slices in the toaster or place the slices on a half sheet baking pan and broil in the oven until crisp and dry but not browned, about 1 minute per side. Rub one side of each toast with the garlic clove and set aside.
Finish the soup: Add ½ teaspoon of sherry to the bottom of four oven safe ramekins, and ladle the soup on top. Top each serving of soup with two garlic-rubbed toasts. Divide the cheese among the servings, covering the bread and some of the soup. Carefully broil the tops of the soup with a chef's torch until cheese is melted and bubbling, or place the ramekins on the half sheet baking pan and transfer to oven to broil and melt the cheese, 4 to 8 minutes. See Recipe Notes for finishing the soup in non-oven-safe soup bowls.
Serve and enjoy your afternoon's labors!
The soup can be made up to 3 days ahead (without toasts or cheese) if refrigerated, or up to 6 months ahead if frozen. Toasts can be made (without cheese) and kept sealed at room temperature for up to 3 days.I use 3 tablespoons of ghee (clarified butter) for cooking the onions, and two tablespoons of butter to finish the soup instead of using all butter. Ghee has a higher smoke point than butter, so it's less lightly to burn while the onions are cooking down.You can substitute Cognac or even Cointreau for the sherry, if desired.If you don't have oven-safe ramekins, then top each garlic-rubbed toast with some cheese and melt in the broiler, about 2 minutes. Divide the sherry and soup among the bowls, sprinkle with the remaining cheese, and top each serving with two cheese toasts.If you want another use for the chef's torch, you can make a foolproof Slow Cooker Crème Brûlée while the onions are cooking down. Low and slow heat is ideal for baking custard gently. Then adding a crunchy sugar crust makes for a winning treat!