Halve, peel, and thinly slice all of the onions. Drizzle 2 tablespoons of the olive oil over the bottom of the slow-cooker's crock. Add about half of the onions, sprinkle about half of the salt over the onions, drizzle another 2 tablespoons of the olive oil, and repeat with the remaining onions, salt, and olive oil. Sprinkle the thyme over everything and then use tongs to toss the contents so the onions are evenly coated with oil. Place the lid on the crock, turn the heat to HIGH, and let it cook for 9-10 hours, stirring and flipping the contents from bottom to top after 1 hour and again after 3 hours. After those stirs, you should not need to keep agitating the contents of the slow cooker unless your cooker runs hot. If you know it runs hot, it wouldn't hurt to move the onions around every couple of hours to prevent scorching. When the onions are deep golden brown to medium brown, turn the slow-cooker off and portion out the onions into freezer safe containers. I tend to do one batch big enough for French Onion Soup, and two smaller containers for pizzas, flatbreads, sandwiches, and whatnot.

Preheat the oven to 400°F.

You can either remove half of the caramelized onions from the slow cooker and finish making the soup in the crock or you can transfer half of the caramelized onions to a pot to make the soup. Either way, add the beef and chicken broths to half of the caramelized onions over HIGH heat.

While the soup heats, drizzle the bread slices with olive oil on both sides. Depending on the size and shape of the bowls you will be using to serve the soup, you may have to trim the bread to fit. Toast the bread in the preheated oven for about 6 minutes, flipping halfway through the time, or until golden brown and crisp on both sides. Set aside.

When the soup is steaming hot, but not boiling, taste the broth and adjust with salt and pepper if necessary. Ladle the soup into broiler-safe bowls, top each bowl with one or two croutons. Top each crouton with a slice of Swiss cheese or a generous handful of grated Swiss cheese. Put the bowls under the broiler until the cheese is melted, bubbly, and browned in places. Serve immediately.