• 115 g unsalted butter
• 2 tbsp olive oil
• 450 g onions, thinly sliced (approximately 3 medium onions)
• 1,200 ml beef stock (made with AquaTru-filtered water if using a cube or concentrate)
• 2 tbsp dry sherry
• 1 tsp dried thyme
• Salt and pepper, to taste
• 4 slices French bread
• 4 slices provolone cheese
• 2 slices Swiss cheese, diced
• 25 g Parmesan, grated

1. Melt the butter with the olive oil in a large stockpot over a medium heat. Add the onions and stir continually until tender and translucent. Do not let them brown.
2. Add the beef stock, sherry, and thyme. Season with salt and pepper, then simmer for 30 minutes. Meanwhile, preheat the grill.
3. Ladle the soup into ovenproof bowls and place a slice of bread on top of each (break it into pieces if you prefer). Top each slice with a slice of provolone, half a slice of diced Swiss cheese, and 1 tbsp Parmesan. Place the bowls on a rimmed baking tray.
4. Grill until the cheese bubbles and browns slightly, 2 to 3 minutes. Serve hot.

Tip: Gruyère is the traditional choice if you would like to swap the cheeses. Store leftover soup without the bread in an airtight container in the fridge for up to 4 days, or freeze for up to 3 months. Reheat gently on the hob and add fresh bread and cheese before grilling.