Ingredients

• 1 lb spaghetti
• 1/3 cup olive oil
• 4 cups chopped scallions (from 4 bunches)
• 2 garlic cloves, finely chopped
• 1 teaspoon finely grated fresh lemon zest
• 3/4 teaspoon salt
• 1/2 teaspoon black pepper
• 1 oz finely grated Parmigiano-Reggiano (1/2 cup)

Instructions

1. Cook spaghetti in a 6- to 8-quart pot of boiling salted water until al dente.
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. 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. Drain pasta in a colander and return to pot.
5. Toss pasta with scallion purée, cheese, and salt and pepper to taste over moderate heat until pasta is well coated.