Spaghetti with Scallion Sauce

Directions

1. 1.

   Cook spaghetti in a 6- to 8-quart pot of boiling salted water until al dente.

2. 2.

   While pasta cooks, heat 3 tablespoons olive oil in a 12-inch heavy skillet over moderate heat until hot but not smoking, then cook scallions and garlic, stirring occasionally, until scallions are softened and garlic begins to turn golden, about 5 minutes.

3. 3.

   Transfer scallion mixture to a blender and add zest, salt, pepper, remaining olive oil, and 1/2 cup pasta-cooking water, then purée until smooth. (Use caution when blending hot liquids.)

4. 4.

   Drain pasta in a colander and return to pot.

5. 5.

   Toss pasta with scallion purée, cheese, and salt and pepper to taste over moderate heat until pasta is well coated.