Toss some onions in the slow cooker and come back to a timeless favorite!

Place the sliced onions, butter, sugar, salt, and rosemary sprig in the bottom of a 6-quart slow cooker.

Cover and cook on high for 7 hours until the onions are soft and caramelized.

Add the beef broth, white wine, thyme sprigs, and pepper to the slow cooker.

Cover and cook on high for another hour to let the flavors meld together.

Preheat your broiler.

Ladle the soup into ovenproof mugs.

Top each mug with a slice of French bread or a handful of croutons.

Add slices of Gruyere cheese on top.

Place the mugs under the broiler until the cheese is bubbly and golden, about 2-3 minutes.

Serve hot and enjoy.

For a vegetarian version, substitute the beef broth with vegetable broth. You can also experiment with different types of cheese, such as Emmental or Comté, for a unique flavor. If you prefer a deeper flavor, try adding a splash of brandy or sherry along with the wine. For a gluten-free option, use gluten-free bread or omit the bread entirely and top with extra cheese.