In a Dutch oven or soup kettle, saute onions over medium heat in oil for 10-12 minutes or until golden brown. Remove and set aside.
In the same kettle, brown beef; drain. Add the water, potatoes, vinegar, bouillon, salt, pepper and reserved onions; bring to a boil. Reduce heat; cover and simmer for 30 minutes or until potatoes are tender.
Stir in parsley. Ladle into ovenproof bowls. Top each with Swiss cheese, a slice of bread and Parmesan cheese. Bake at 375° for 10 minutes or until cheeses are bubbly.