10-12 medium white or yellow onions, sliced thin (about 12 cups), divided
6 tablespoons butter, divided
4 cups low-sodium chicken stock
1/2 cup dried porcini mushrooms
1 cup low-sodium beef stock
2 teaspoons Worcestershire sauce
1/2 teaspoon dried thyme (or 2 sprigs fresh thyme)
1/4 cup dry sherry
2 teaspoons sherry vinegar (optional)
4 thin slices French or Italian bread
2 ounces Gruyere or Emmenthal cheese, coarsely grated
French onion soup
Place a large saute pan with a lid over low heat. Add half the butter and heat until butter is melted. Add about two thirds of the onions and stir to coat. Sprinkle with salt and cover the pan. Cook on lowest heat, covered, for at least an hour, stirring every 15 minutes or so. (Because the moisture level in onions can vary substantially, this process can take longer; I’ve had batches take 2 hours.) The onions will soften, then slowly turn golden and then amber in color. They’ll lose volume and continue to darken, until you should end up with about 1-1/2 of cup of deep amber onions.
Meanwhile, heat the chicken stock (either on the stove or in the microwave) just until simmering. Remove from heat and add the porcini. Let steep for 10 to 20 minutes. Pour the stock through a fine-mesh sieve and discard the mushrooms.
In a large saucepan, melt the remaining butter over medium high heat. Heat until butter stops foaming and then add the remaining third of the onions. Sprinkle with salt and cook, stirring often, until browned but still slightly firm.
Remove the onions from the pan. Pour the sherry into the pan and stir to scrape up the browned bits from the bottom of the pan. Let most of the sherry evaporate and then add the chicken stock, beef stock, thyme and Worcestershire sauce. When the onions from step 1 are done, add them to the soup and simmer for 15 to 20 minutes. Taste and adjust seasoning, adding the sherry vinegar if the soup is too sweet. Just before serving, add the onions from step 3.
When ready to serve, turn the broiler on. Reserve 1/4 cup of the cheese, and sprinkle the rest over the two slices of bread. Place the bread slices on a sheet pan and put it under the broiler until cheese melts (2-3 minutes).
Divide the reserved cheese between four soup bowls. Ladle the soup into the bowls and float a bread slice on top of each bowl.