In a large skillet over medium heat, melt 4 Tablespoons salted butter. Add the 5 medium sweet onions, thinly sliced and 1/2 teaspoon black pepper to the skillet and coat them with the butter.
Continue to cook, occasionally stirring, until caramelized and browned, 45-60 minutes. During the last 20 or so minutes, the onions will have a tendency to burn if you do not stir them often enough, so make sure to keep them moving and keep an eye on them.
Add 4 cloves garlic, minced and cook until fragrant, 30 seconds.
Deglaze the pan with 1/3 cup cooking sherry and take off the heat.
Add the cooked onions to a 6-8 quart slow cooker.
Add 64 ounces beef stock, 1 Tablespoon Worcestershire sauce, 6 sprigs fresh thyme and 1 bay leaf to the slow cooker. Stir to combine and place on high for 3-4 hours or low 6-8 hours. After cooking, remove the sprigs of thyme and the bay leaf.
When you are ready to serve, preheat the oven to 425°F.
Lay the 1 French baguette, cut into ½ inch slices on a large sheet tray and brush both sides of the slices with ¼ cup olive oil, it seems like a lot, but use it all.
Bake for 5-6 minutes per side, flipping once until lightly golden and toasted.
Place the soup into a broiler-safe crock or bowl, leaving about ½ inch at the top of the bowl.
Place a few of the toasted bread slices on top, and top generously with 14 ounces gruyere cheese, shredded, try to cover up the toast as much as possible. Put the bowls on a sheet tray to catch any drips.
Place the bowls under the broiler until dark golden brown, let cool because it is extremely hot, and serve.
Notes
It is not too salty from using both salted butter and beef stock. But, if you are watching your salt intake, start with both unsalted stock and butter, then taste and adjust the seasoning.
You can use 1 teaspoon of dried thyme if you don’t have fresh.
You absolutely have to make sure the bowl you use to broil the cheese is rated for use under the broiler. If not, it may crack or damage the bowl. If you don’t have an oven safe bowl, you can just melt the cheese directly on the baguettes in the oven then transfer to the serving bowls.
If you are not planning to eat the entire soup in one sitting, I suggest only making enough croutons/toasts for how much you are planning to eat. They may become soggy if left in an airtight container at room temperature for multiple days.
It seems like a lot of cheese for the recipe, you really need to add a generous portion to each bowl to get the signature “french onion soup” look.
Keep leftovers in the fridge for up to 5 days, freeze for up to 3 months