French Onion Soup

Ingredients

• 4 lb yellow onions (about 6 large)
• 2 tbsp grapeseed oil
• 2 tbsp unsalted butter
• 1/2 tsp kosher salt
• 1/4 cup dry sherry
• 8 cups dark beef stock (or dark chicken stock)
• 1 tsp fresh thyme leaves
• 1 bay leaf
• kosher salt (to taste)
• freshly ground black pepper (to taste)
• 12 to 16 slices baguette
• 12 oz Gruyère cheese*
• 2 tbsp apple cider vinegar (or to taste)