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1
Heat the 2 tablespoons olive oil in a large skillet over moderate heat.
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2
Add the garlic and cook until the cloves start to color.
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3
Add the sage and let the leaves sizzle for 10 seconds, then add the thinly sliced onions and salt and cook for 2 minutes.
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4
Add the water, cover, reduce the heat to low, and cook until the onions are quite soft, about 15 minutes.
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5
Uncover and cook until most of the liquid evaporates.
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6
Do not allow the onions to color.
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7
Let cool to room temperature.
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8
With a boning knife, carefully cut between the muscles to open up the shoulder so that it lies flat like a book.
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9
(You can also ask the butcher to do this for you.)
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10
Rub the surface with 1 tablespoon of the spice rub, then spread the cooled onions on top.
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11
Reshape the shoulder to enclose the onions, as if you were closing the book.
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12
Tie the pork lightly with kitchen string, taking care not to squeeze the onions out.
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13
Rub it all over with olive oil, then with the remaining 3 tablespoons spice rub.
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14
(You can refrigerate the pork overnight at this point, but bring it to room temperature before continuing.)
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15
Preheat the oven to 275F.
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16
Line a roasting pan or heavy rimmed baking sheet with heavy-duty aluminum foil.
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17
Put the celery, carrots, and onion halves in the roasting pan to make a vegetable rack and top with the pork.
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18
Roast the pork until it is fork-tender, about 7 hours.
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19
Let cool for at least 15 minutes before removing the string.
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20
Discard the vegetables; transfer the pork to a platter.
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21
To serve, pull the roast apart into large chunks with a meat fork rather than carving it into neat slices.
