1 tbsp olive oil
1kg onion, 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
Melt 50g butter with 1 tbsp olive oil in a large heavy-based pan. Add 1kg halved and thinly sliced onions and fry with the lid on for 10 mins until soft.
Sprinkle in 1 tsp sugar and cook for 20 mins more, stirring frequently, until caramelised. The onions should be really golden, full of flavour and soft when pinched between your fingers. Take care towards the end to ensure that they don’t burn.
Add 4 thinly sliced garlic cloves for the final few mins of the onions’ cooking time, then sprinkle in 2 tbsp plain flour and stir well.
Increase the heat and keep stirring as you gradually add 250ml dry white wine, followed by 1.3l hot strongly-flavoured beef stock. Cover and simmer for 15-20 mins.
To serve, turn on the grill, and toast 4-8 slices French bread. Ladle the soup into heatproof bowls.
Put a slice or two of toast on top of the bowls of soup, and pile on 140g finely grated gruyère cheese. Grill until melted. Alternatively, you can complete the toasts under the grill, then serve them on top.