(Use your own preferred method for caramelized onions, if you prefer). Set the oven to 400 degrees Fahrenheit. In a Dutch oven, combine the sliced onions, olive oil, and salt. Cover with the lid and bake for 1 hour. Remove from the oven and stir, especially making sure to scrape the sides.
Return onions to the oven, with the lid slightly ajar to let steam escape. Cook for another 60 minutes, then stir again and return to the onion. Continue cooking for about 30 more minutes, or until onions are soft and golden brown.
Continue making the soup using the caramelized onions in the same pot, or store them in the refrigerator (including oil) for up to 3 days.
Cook the soup:
Move the pot with the caramelized onions to the stove over medium heat (medium-low if using a gas stove). If using stored onions from the refrigerator, add the additional 2 tsp of olive oil to help keep them from sticking. Add the black pepper.
Cook the onions until nicely browned, stirring frequently and scraping the bottom and sides, about 10-12 minutes. Turn the heat to medium, stir in 1/2 cup water and scrape the bottom and sides well. Then, stir in the dry sherry and cook until the liquid is mostly evaporated, about 3-4 minutes.
Add the broth, thyme, bay leaf, salt, brown sugar, and 2 cups of water. Bring to a boil, then reduce the heat to low, cover, and simmer for 30 minutes. Season to taste with additional salt and pepper if needed. Discard the thyme and bay leaf. Prepared soup keeps well in the fridge for up to 4-5 days.
Make the croutons:
Preheat the oven to 350 degrees Fahrenheit.
In a mixing bowl, combine all of the ingredients except for the bread. Stir into an even mixture, then add the bread to the bowl and toss to combine. Use your hands to massage the seasoning into the bread.
Transfer the seasoned bread cubes to a baking sheet and bake for about 15 minutes, or until golden brown and slightly crisp. After cooling, store any leftovers in an airtight container at room temperature for up to 3 days.
MAKE AHEAD: The onions, croutons, and homemade broth (if using) can all be made in advance and stored for a couple of days, which means that soup day is actually pretty quick.BREAD CHOICE: I always use my boyfriend's homemade 50% whole wheat sourdough for these croutons, and it's simply amazing. Any type of sourdough will be great here, but do try to use bread that isn't pre-sliced; it will be much more moist and flavorful.CARAMELIZED ONIONS: Use a stovetop caramelized onion method if you prefer or don't have time to use the oven method. However, be sure to use the full quantity of olive oil listed in the recipe, as it is necessary for the flavor and texture of the soup. BROTH: My homemade vegan beef broth is what makes this recipe truly stupendous. However, if you need to use a premade option, I urge you use to use something labeled as beef-style broth, rather than vegetable broth. A vegetable broth would need to be doctored up with high-glutamate ingredients to provide the desired flavor.NUTRITION: Recipe nutrition facts include the croutons.