In a 6-quart slow cooker, layer in the sliced Vidalia onions along with the melted unsalted butter, granulated sugar, kosher salt, and freshly cracked black pepper. Toss everything together until the onions are well-coated. Add in the fresh thyme and bay leaf for an aromatic touch, ensuring an even distribution.

Cover the slow cooker and set it to HIGH. Allow the onions to cook for 6-7 hours, stirring once halfway through to promote even caramelization.

After the onions have reached that beautiful caramelized state, remove the thyme and bay leaf. Pour in the lower-sodium beef broth and sherry vinegar, stirring to combine. Allow the mixture to cook on HIGH for an additional 30 minutes.

Preheat your oven to the broil setting. Carefully ladle the hot soup into ramekins placed on a baking sheet.

On each ramekin, place two toasted baguette slices. Generously sprinkle with Gruyère cheese to create a gooey, bubbling top. Slide the baking sheet into the oven and broil for about 3 minutes.

Carefully remove the ramekins from the oven. Sprinkle lightly with fresh thyme leaves and a dash of pepper for added flavor.