3 pounds sweet onions, thinly sliced 4 tablespoons unsalted butter, melted 2 teaspoons brown sugar 2 teaspoons sherry vinegar 1 tablespoon Worcestershire sauce 5 fresh thyme sprigs 3 bay leaves 6 cups beef stock Kosher salt and freshly ground black pepper, to taste 1 (12-ounce) French baguette, sliced 2 cups Gruyère cheese, shredded

Add the thinly sliced onions to a 6-qt slow cooker. Stir in butter, brown sugar, 1 teaspoon salt, sherry vinegar and Worcestershire sauce.
Cover and cook on high heat for 8-9 hours or on low for about 12-14; or until onions are caramelized and browned. Make sure to stir halfway, as needed, to prevent burning.
Stir in beef stock, season with salt and pepper, to taste and add thyme sprigs and bay leaves. Cover and cook on high heat for 2-3 hours or on low for 4-5 hours. When ready discard thyme and bay leaves.
Preheat oven to broil.
Place baguette slices onto a baking sheet and broil until golden brown on both sides, about 1-2 minutes per side.
Divide soup into ovenproof bowls, place the bowls on a baking sheet. Top with toasted baguette slices, about two slices per bowl, the bread should cover the soup; top with lots of shredded cheese.
Place into the oven and broil until the cheese is bubbly and golden brown, about 2 minutes.
Serve immediately.