Step 1

Make the pesto: Puree the garlic scapes, pistachios, Parmesan, 1/2 teaspoon salt, and 1/4 teaspoon pepper in a food processor until very finely chopped. With the motor running, slowly pour the oil through the opening. Season the pesto with salt and pepper to taste. (The pesto keeps in the fridge, covered, for 1 week or frozen for a month.)

Step 2

In a large pot of heavily salted boiling water, cook the spaghetti until al dente. Reserve 1 cup of the pasta-cooking water, then drain the pasta. Whisk together 2/3 cup of the pesto and the reserved pasta water and toss with the pasta. Season with salt and pepper to taste and serve right away.