Heat a large Dutch oven over medium heat. Add 6 tablespoons of the butter and let it melt. Stir in the onions and the thyme and season with salt and pepper, to taste. Cook, stirring occasionally, until the onions caramelize, about 40 to 45 minutes.
Meanwhile, put the mushrooms in a small pot and cover with a couple of cups of water. Bring to a boil over high heat, then reduce the heat to low and simmer to reconstitute the mushrooms.
Add the sherry to the pot with the onions, to deglaze the pan, then stir in the wine. Remove the mushrooms from their broth with a slotted spoon to a cutting board. Chop them, and add them to the onions. Add all but the last 1/2 cup of the mushroom liquid to the soup, about 1 cup. Add herb bundle and the beef stock to the pot and simmer the soup for 30 to 45 minutes. Cool and refrigerate for a make-ahead meal. Reheat over medium heat, covered, and prepare the croutons when ready to serve.
Heat the oven to 425 degrees F. Toast the bread on a rack over a baking sheet, until deep golden brown. Butter the bread, using the 2 tablespoons of remaining butter and liberally rub them with the cut garlic. Top the toasts with the cheeses and return them to the oven until the cheese is melted and bubbly. Toasts may be arranged on top of crocks of soup, put on baking sheet, then topped with cheese and baked to brown, for a more dramatic presentation.
Ladle the soup into serving bowls and serve with the cheese toasts.