Cut off the stem and root ends of the onions, then halve then lengthwise, and peel away the dried brown skin. Cut the onions into 1/4-inch slices. It is important to have all the onion slices roughly the same size so they cook evenly.
In a large pot over low heat, melt the butter. Add the sliced onions and cook over low heat, stirring frequently, until they are deeply browned, approximately 2 hours. I like to use my oven method for caramelizing onions as this method cuts down on the cooking and stirring time required when you caramelize on your stove top. Check out Baked Caramelized Onions.
Using a wooden spoon, stir in the dry sherry (or other alcohol), balsamic vinegar, Worcestershire sauce, thyme, and a pinch of salt and pepper. Cook over low heat until the liquid is reduced slightly, about 2 minutes. Add the chicken broth and let reduce slightly, approximately 5 minutes.
Preheat your oven broiler.
Divide the caramelized onion mixture between two shallow, heat-proof bowls or serving dishes (such as gratin dishes). Cover each with the grated Gruyere cheese, dividing it equally between the dishes.
Broil until the cheese is bubbly and golden brown, about 4 minutes. Remove from oven and garnish with the parsley. Let cool slightly before servings.
Serve with the baguette slices for dipping.
French Onion Fondue Recipe: https://whatscookingamerica.net/appetizers/frenchonionfondue.htm