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 oil
1/2 teaspoon black pepper
1 lb dried linguine
Finely chop 1/4 cup pecans (preferably with a knife).
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.
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.