Put the oil in a large heavy-based saucepan over a medium heat. Add the bacon and onions and cook for about 5 minutes until the onion is soft and pale golden.
Pour in the stock and 600 ml (1 pint) of the measurement water, add the potatoes and bring to the boil. Reduce the heat, cover and simmer for about 15 minutes until the potatoes are tender.
Whisk the flour with the remaining measurement water in a small bowl then stir the mixture into the soup. Cover and simmer, stirring frequently, for another 5 minutes.
Blend the Gruyère with 300 ml (½ pint) of the soup in a blender or food processor until smooth. Return to the pan and add the sherry and Worcestershire sauce. Season with salt and pepper, bearing in mind the saltiness of the bacon and cheese. Gently simmer for 3–5 minutes.
Stir in the parsley, if using, and serve immediately in warm soup bowls with grilled cheese on toast sprinkled with a little Worcestershire sauce, if liked.