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1
Discard tough ends from yau choy, then chop stems into 1-inch pieces and coarsely chop leaves.
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2
Cook stems in a 6- to 8-quart pot of boiling salted water until just tender, about 4 minutes.
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3
Stir in leaves and cook 1 minute.
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4
Transfer yau choy with a slotted spoon to a colander (keep water simmering for pasta), then rinse under cold water to stop cooking and drain again.
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5
Do not squeeze out excess water.
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6
Cook bacon in oil in a 12-inch deep heavy skillet over moderate heat, stirring, until golden brown and crisp, 5 to 7 minutes.
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7
Add onion and cook, stirring, until golden, about 15 minutes.
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8
Add garlic to onion and cook over moderate heat, stirring, until golden, about 2 minutes.
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9
While onion and garlic are cooking, bring pot of simmering water to a boil.
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10
Add pasta and cook until al dente.
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11
Reserve 1 cup of pasta cooking water, then drain pasta in colander.
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12
Add reserved cooking water to onion and cook, stirring and scraping up brown bits, 30 seconds.
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13
Add yau choy and toss to combine.
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14
Add pasta, cheese, salt, and pepper and cook over moderately high heat, tossing, until well combined, 1 to 2 minutes.
