Caramelized Onion and Bread Soup with Brûléed Blue Cheese

Name: Caramelized Onion and Bread Soup with Brûléed Blue Cheese
Ingredients: 4 1/2 pounds mixed onions (such as 2 large yellow onions, 2 large red onions, and 2 large sweet onions), halved and thinly sliced lengthwise (about 16 cups); 2 tablespoons extra-virgin olive oil; 2 tablespoons unsalted butter; 1/4 cup oloroso sherry; 6 cups homemade or store-bought low-sodium vegetable broth; 8 thyme sprigs; 1 tablespoon kosher salt, plus more to taste; 3/4 teaspoon freshly ground black pepper, plus more to taste; 2 teaspoons sherry vinegar; 6 day-old whole-grain rustic bread slices, halved; 4 ounces Stilton cheese, thoroughly chilled and thinly sliced with a wire cheese slicer or crumbled