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1
Heat the oil in a 6-quart Dutch oven over high heat.
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2
Add the oxtail (or beef shoulder) in a single layer (work in batches, if necessary to avoid crowding the pan), and sear until the undersides are brown (do not turn).
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3
Season generously with salt and transfer to a plate.
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4
Coarsely chop two of the onions; add to the pot, along with the celery, carrots, bay leaves and thyme.
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5
Lower heat to medium and cook, stirring occasionally, until vegetables are soft and beginning to caramelize, about 10 minutes.
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6
Return the beef to the pot.
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7
Pour in 8 cups water.
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8
Simmer mixture gently until the meat is very tender, 2 1/2 to 3 hours.
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9
Transfer beef to a bowl to cool for another use.
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10
Strain liquid into a bowl over a fine-mesh sieve; press gently on the solids with the back of a spatula to extract as much flavor as possible.
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11
Discard the solids; you should have about 10 cups broth (add water if necessary to equal 10 cups).
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12
Halve the remaining 6 onions through the root end, then peel and thinly slice them lengthwise.
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13
Melt the butter in the bottom of the Dutch oven over medium heat.
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14
Add the onions and cook, tossing occasionally, until deep golden-brown and caramelized, 45 minutes to 1 hour.
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15
Season with 1 teaspoon salt and black pepper.
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16
Pour in the port and cook, scraping up any browned bits from the bottom of the pan, for 3 minutes.
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17
Pour in the broth and simmer mixture over low heat for 30 minutes.
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18
Season with salt and lemon juice, if desired.
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19
(For a smaller group, you could refrigerate some of the soup and reheat it later.)
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20
While the broth simmers, heat the oven to 350 degrees.
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21
Arrange the bread slices on a baking sheet and toast until golden, about 12 minutes.
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22
Rub the garlic halves over the surface of the bread.
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23
Heat the broiler and arrange a rack 4 to 6 inches from the flame.
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24
Using a cheese slicer, thinly slice 3 ounces of Gruyere.
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25
Coarsely grate the remaining cheese.
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26
Float the broiled bread over the surface of the hot soup.
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27
Layer the cheese slices over the bread; scatter the grated cheese over it.
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28
Transfer the Dutch oven to the oven and broil until cheese is golden and bubbling, 3 to 5 minutes (watch to see that it does not burn).
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29
To serve, use kitchen shears or scissors to cut the bread and cheese into portions.
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30
Ladle soup, bread and cheese into individual bowls.
