4 1/2 lbs. mixed onions, such as sweet onions, 2 large red onions, halved and thinly sliced lengthwise
2 tbsp. extra virgin olive oil
3 tbsp. unsalted butter
1/4 cup cooking sherry
3 cups each of vegetable broth and bone broth
8 fresh thyme sprigs
1 tbsp. kosher salt, to taste
3/4 tsp. freshly ground black pepper
6 slices day old rustic whole grain bread, or bagette
4 oz. each, high quality emmanthaler and jarlsberg swiss cheese sliced with a cheese plane
Combine the onions, oil and butter in a large stockpot over medium-high heat. Toss together with tongs. Cook, stirring occasionally until onions start to caramelize, about 20 minutes. Reduce heat to medium and continue to cook, stirring and scraping browned bits from bottom of pot, until onions are tender and golden in color, about 25 minutes.
Add sherry and cook, stirring to scrape up any remaining browned bits. Simmer over medium heat stirring constantly until sherry is mostly evaporated, 2-3 minutes. Stir in broth, thyme, salt and pepper. Increase heat to high and bring to a boil. Reduce heat to medium-low and simmer, partially covered until flavors are blended, 20-25 minutes. Add additional salt and pepper to taste if needed.
Preheat oven to broil with oven rack 6 inches from heat. Remove and discard the thyme sprigs. Divide soup evenly among 6 oven-proof crocks or bowls and place on rimmed baking sheet. Place a slice of bread on the soup in the bowls. Add cheese slices to cover the bread and the bowls. Broil until melted, bubbly and browned, 2-3 minutes. Serve immediately.