In a large saucepan over low heat, melt butter with olive oil. Cook onions in mixture, stirring occasionally, for 30 minutes, until very soft, translucent and brown on the edges.
Sprinkle the flour over the onions and stir to combine. Pour in beef broth, wine and water with bay leaves. Bring to a boil, then reduce heat and simmer, uncovered, 20 to 30 minutes.
Preheat oven broiler.
Toast the bread slices. Ladle the soup into 6 ovenproof bowls and place the bowls on a baking sheet. Place a toasted slice of bread over each bowl and top with Swiss cheese.
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