1/2 c. unsalted butter
5 lbs. white onions, thinly sliced
¼ c. Amontillado sherry
¼ c. Worcestershire sauce
2 t. dried thyme
1 T. chicken stock concentrate
Salt and freshly ground pepper
2 c. coarsely grated Gruyère cheese
1 t. finely chopped flat-leaf parsley
In a large pot, melt the butter over low heat. Add the onions and about 1 teaspoon salt and cook over low heat, stirring frequently, until they are deeply browned and reduced to marmalade consistency, about 3 hours.
Using a wooden spoon, stir in the sherry, Worcestershire, thyme, chicken stock concentrate, and salt and pepper to taste. Cook over low heat until the liquid is reduced, about 5 minutes.
Preheat the broiler. Divide the caramelized onions between shallow, heatproof bowls or serving dishes (such as gratin dishes or ramekins). Cover with the Gruyère, dividing it equally between the dishes. Broil until the cheese is bubbly and golden brown, about 4 minutes. Garnish with the parsley. Cool slightly; serve with baguettes and sliced apples for dipping.