Preheat the oven to 200°C
Melt the butter in a large, deep pot over a medium heat. Add in the sliced onions and cook, stirring occasionally, until they are caramelised and golden brown, about 25 minutes
Stir in the minced garlic and fresh thyme leaves, then season with salt to taste. Cook for an additional 2–3 minutes
Add in the Calvet Muscadet White Wine and let it reduce by half, then add in the beef stock, balsamic vinegar and Worcestershire sauce before seasoning with salt and pepper as needed. Leave to simmer for 15 minutes
Transfer the soup to a medium sized roasting tray, and layer the baguette slices over the top of the onions and broth, making sure they are just slightly submerged, then sprinkle over the grated Gruyère
Place the roasting tray in the oven and bake for 20 minutes until the cheese has melted and the baguette slices are golden brown
To serve, remove the tray bake from the oven and garnish with a few sprigs of fresh thyme
Ladle the French onion soup into bowls and serve while hot. Enjoy with a glass of Calvet Muscadet White Wine