There are two essential components of a good French onion soup. The first is the stock. Your soup will only be as good as the stock you are using. This soup traditionally is made with beef stock, although you can use vegetable stock.
The second most important element is to properly caramelize the onions

Ingredients:

50g / 2oz butter
1 tbsp extra virgin olive oil
Dash Worcestershire sauce
1kg / 2lb onions, thinly sliced
1 tsp sugar
4 garlic cloves, crushed and finely chopped
2 tbsp plain flour
250ml / 9 floz sherry (or dry white wine)
1.2l / 40 floz strongly-flavoured beef stock
To garnish French bread & grated Emmental cheese.

Method:

Melt the butter with the oil and Worcestershire sauce in a large heavy-based pan. Add the onions and briskly fry for a 1-2 mins, reduce heat put the lid and continue to cook for 8 minutes or until soft. Sprinkle in the sugar and cook for 20 minutes more, stirring frequently, until caramelised. Take care towards the end to ensure that they don’t burn
Add the garlic for the final few minutes of the onions’ cooking time, then sprinkle in the flour and stir well. Increase the heat and keep stirring as you gradually add the wine, followed by the hot stock. Cover and simmer for 15-20 minutes

To serve, turn on the grill, and toast the bread. Ladle the soup into heatproof bowls. Put a slice or two of toast on top of the bowls of soup, and pile on the cheese. Grill until melted. Alternatively, you can complete the toasts under the grill, then serve them on top