1. Slice onions very thinly preferably using a mandoline or food processor. Heat butter in a large saucepan over medium heat until melted. Add onions all at once; adjust heat upwards towards medium-high as needed to cook onions. 2. Stirring occasionally, sweat onions for 25 minutes or until onions are pale golden, greatly reduced in volume and a deep brown slick is forming on pan bottom. (Watch carefully towards end of cooking time to prevent burning.). 3. Stir in broths, wine, thyme and bay leaf. Bring to a boil; cover and reduce heat. Simmer for 20 to 25 minutes. Discard bay leaf and thyme sprig. Stir in vinegar and black pepper; taste and add pinches of salt if needed. Stir in parsley. 4. Slice bread on a slight diagonal into ½-inch-thick slices, 2 slices for every portion intended to be served. Lightly butter 1 side of each slice. Slice cheeses. Make a sandwich, buttered-sides out, with a slice of each kind of cheese inside (trim cheese to fit as needed). Take remaining cheese slices and scraps and cut crosswise into long thin pieces. 5. Heat soup until very hot, but not boiling. Using a large frying pan or griddle, slowly grill sandwiches over medium-low heat for 10 minutes per side or until bread is golden on both sides and interior cheese is melted. Heap remaining cheese pieces on top of sandwiches while still in pan; broil 1 to 2 minutes or until melted. 6. Ladle soup into bowls. Slide a sandwich into each portion and serve immediately sprinkled with additional chopped parsley. ---------------------------------------------------------------------------
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