How to Make It
Combine onions, oil, and butter in a stockpot over medium-high; toss together. (Tongs are a great tool to use for tossing this many onions.) Cook, stirring occasionally, until onions start to caramelize, about 20 minutes. Reduce heat to medium, and continue to cook, stirring and scraping any browned bits from bottom of pot with a wooden spoon, until onions are tender and caramelized, about 25 minutes.
Add sherry, and cook, stirring to scrape up any remaining browned bits. Simmer over medium, stirring constantly, until sherry is mostly evaporated, 2 to 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 marry, 20 to 25 minutes. Stir in vinegar, and cook 1 minute; add salt and pepper to taste.
Preheat oven to broil with oven rack 6 inches from heat. Remove and discard thyme sprigs. Divide hot soup evenly among 6 ovenproof crocks or bowls, and place on a rimmed baking sheet. Place 2 bread pieces on each bowl, and gently push them down until half- submerged but still at top of soup. Add cheese slices (or a handful of crumbles) to each bowl. Broil until melted, bubbly, and browned in spots, 2 to 3 minutes.
The soup, cooked through step 2, will keep, covered, in the refrigerator up to 3 days. It can be frozen in an airtight container up to 3 months.