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1
Put oven rack in middle position and preheat oven to 350F.
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2
Cut out a round of parchment paper to fit just inside a 5- to 6-quart heavy ovenproof pot, then set round aside.
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3
Blanch kale in a 6- to 8-quart pot of boiling salted water in 2 batches, 2 minutes per batch, transferring with tongs to a colander as blanched, then drain well.
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4
Warm oil in 5- to 6-quart pot over moderate heat, then add pancetta and cook, stirring frequently, until crisp, 10 to 15 minutes.
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5
Transfer pancetta with a slotted spoon to a plate and reserve, discarding all but 1/4 cup of fat in pot (if necessary).
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6
Tie thyme, bay leaf, and parsley together with kitchen string to make a bouquet garni and add to pot along with onions and garlic.
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7
Cook over moderate heat, stirring frequently, until onions are golden brown, about 20 minutes.
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8
Return pancetta to pot then add butter and heat, stirring occasionally, until melted.
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9
Stir in kale and add enough stock to cover three fourths of kale.
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10
Increase heat to high and bring to a simmer.
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11
Cover kale directly with parchment round, then transfer pot to oven and braise, stirring once or twice, until kale is very tender, about 45 minutes.
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12
Transfer pot to stovetop and discard parchment, then boil mixture, stirring occasionally, until almost all of liquid is evaporated but kale is still moist, about 35 minutes.
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13
Discard bouquet garni and garlic and season kale with salt and pepper.
