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1
Slice onions very thinly preferably using a mandoline or food processor.
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2
Heat butter in a large saucepan over medium heat until melted.
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3
Add onions all at once; adjust heat upwards towards medium-high as needed to cook onions.
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4
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.
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5
(Watch carefully towards end of cooking time to prevent burning.
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6
).
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7
Stir in broths, wine, thyme and bay leaf.
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8
Bring to a boil; cover and reduce heat.
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9
Simmer for 20 to 25 minutes.
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10
Discard bay leaf and thyme sprig.
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11
Stir in vinegar and black pepper; taste and add pinches of salt if needed.
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12
Stir in parsley.
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13
Slice bread on a slight diagonal into 1/2-inch-thick slices, 2 slices for every portion intended to be served.
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14
Lightly butter 1 side of each slice.
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15
Slice cheeses.
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16
Make a sandwich, buttered-sides out, with a slice of each kind of cheese inside (trim cheese to fit as needed).
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17
Take remaining cheese slices and scraps and cut crosswise into long thin pieces.
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18
Heat soup until very hot, but not boiling.
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19
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.
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20
Heap remaining cheese pieces on top of sandwiches while still in pan; broil 1 to 2 minutes or until melted.
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21
Ladle soup into bowls.
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22
Slide a sandwich into each portion and serve immediately sprinkled with additional chopped parsley.
