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•   3/4 cup pecans (3 oz), toasted
  1 large garlic clove
  1/2 teaspoon salt
  10 oz arugula, coarse stems discarded
  1/2 cup finely grated Parmigiano-Reggiano (1 1/2 oz)
  1/2 cup olive o 3/4 cup pecans (3 oz), toasted
  1 large garlic clove
  1/2 teaspoon salt
  10 oz arugula, coarse stems discarded
  1/2 cup finely grated Parmigiano-Reggiano (1 1/2 oz)
  1/2 cup olive o

Instructions

1. Finely chop 1/4 cup pecans (preferably with a knife).

2. Mash garlic to a paste with salt using a mortar and pestle (or mince and mash with a large heavy knife). Blend remaining 1/2cup pecans, arugula, cheese, oil, pepper, and garlic paste in a food processor until smooth, about 1 minute.

3. Cook pasta in a 6- to 8-quart pot of boiling salted water until al dente, 10 to 12 minutes. Ladle out and reserve 1 1/2 cups cooking water. Drain pasta in a colander, then return to pot and toss with pesto, 1/2 cup cooking water, and chopped pecans, adding more cooking water as necessary if pasta seems dry.

4. Cooks' note:
   
5. • You can substitute 2 cups fresh flat-leaf parsley for the arugula, but then you should use only 1/3 cup olive oil (instead of 1/2 cup) in the pesto.