Martha's French onion soup with cheesy pancetta croutons

1. Peel and thinly slice the onions. Melt the butter with the oil over a low heat in a large, heavy-bottomed saucepan or casserole dish that has a lid. Add the onions, stir to coat in the fat and cover with the lid. Allow to sweat, stirring often, for 20 minutes.

2. Remove the lid and turn the heat up to medium. Cook the onions for a further 40 minutes until caramelised, stirring often to stop them from burning. The onions will have turned a dark, nutty colour when they are ready.

3. Stir in the flour and cook for a minute, then pour in the wine. Use your spoon to scrape the bottom of the pan so all the delicious browned bits are mixed into the wine. Turn up the heat, bringing the mixture to a simmer, stirring, to evaporate the alcohol. Pour in the stock and add the Marmite, if using. Stir well to combine, then cover and simmer for 30 minutes.

4. Toast the baguette slices under the grill. Sprinkle with the cheese and top each one with a slice of pancetta, then grill for a few minutes until the cheese is bubbling and the pancetta is crisp. Season the soup to taste, then ladle into bowls. Top each bowl with a cheesy crouton and serve immediately.

Cook’s tip This soup will keep in the fridge for up to 2 days or can be frozen and defrosted before reheating. Make the croutons at the time they are needed.