Penne with Yau Choy

Discard tough ends from yau choy, then chop stems into 1-inch pieces and coarsely chop leaves. Cook stems in a 6- to 8-quart pot of boiling salted water until just tender, about 4 minutes. Stir in leaves and cook 1 minute.

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. Do not squeeze out excess water.

Cook bacon in oil in a 12-inch deep heavy skillet over moderate heat, stirring, until golden brown and crisp, 5 to 7 minutes.

Add onion and cook, stirring, until golden, about 15 minutes.

Add garlic to onion and cook over moderate heat, stirring, until golden, about 2 minutes.

While onion and garlic are cooking, bring pot of simmering water to a boil.

Add pasta and cook until al dente. Reserve 1 cup of pasta cooking water, then drain pasta in colander.

Add reserved cooking water to onion and cook, stirring and scraping up brown bits, 30 seconds.

Add yau choy and toss to combine.

Add pasta, cheese, salt, and pepper and cook over moderately high heat, tossing, until well combined, 1 to 2 minutes.