French onion soup

INGREDIENTS

• 50g butter
• 1 tbsp olive oil
• 1kg onions , halved and thinly sliced
• 1 tsp sugar
• 4 garlic cloves , thinly sliced
• 2 tbsp plain flour
• 250ml dry white wine
• 1.3l hot strongly-flavoured beef stock
• 4-8 slices French bread (depending on size)
• 140g Gruyère , finely grated