Preheat your broiler.
Heat the oil in a large pot.
Add onions and cook while stirring until golden (about 10-15 minutes).
Increase heat and add chicken broth, Worcestershire sauce, bouillon, and sherry.
Bring to a boil then simmer over low heat for 5 minutes.
Add Parmesan cheese, salt and pepper and simmer for another 3 minutes.
Pour soup into 4 heat proof bowls, top each with Swiss cheese and put under the broiler until hot and bubbly
Sprinkle shredded Swiss on top of soup and serve.
Pro tip - if I'm serving this to any kids, I use the second option as the crocks get quite hot under the broiler.