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1
Cut asparagus into 1-inch pieces, reserving tips separately.
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2
Cook asparagus stems in 5 to 6 quarts boiling water with 2 tablespoons salt until very tender, 6 to 8 minutes.
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3
Transfer with a slotted spoon to a colander, reserving cooking water in pot, and rinse under cold water.
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4
Drain asparagus well and transfer to a food processor or blender.
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5
Cook asparagus tips in same boiling water until just tender, 3 to 5 minutes.
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6
Transfer with slotted spoon to colander, reserving boiling water in pot, and rinse under cold water.
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7
Drain tips well.
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8
Puree asparagus stems with zest, oil, and 1/2 cup asparagus cooking water.
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9
Transfer sauce to a 4-quart saucepan.
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10
Cook pasta in boiling asparagus cooking water until it still offers considerable resistance to the tooth, around three fourths of the recommended cooking time.
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11
Reserve 2 cups cooking water and drain pasta.
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12
Add pasta, asparagus tips, and 1/2 cup reserved water to asparagus sauce and cook over high heat, stirring, 3 to 5 minutes, or until pasta is almost al dente and sauce coats pasta.
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13
Add more cooking water, 1/4 cup at a time, until sauce coats pasta but is a little loose (the cheese will thicken it slightly).
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14
Stir in Parmigiano-Reggiano and salt and pepper to taste and cook, stirring, until cheese is melted.
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15
Serve immediately.
