To prepare the penne: In a large saucepan of salted boiling water blanch the sugar snap peas for 45 seconds, or until they are crisp-tender, transfer them with a skimmer to a large serving bowl, and toss them with 1/2 cup of the pesto. In the boiling water cook the pasta until it is al dente, reserve 1/2 cup of the pasta-cooking water, and drain the pasta in a colander. In the bowl with the sugar snap peas toss the pasta with the reserved pasta-cooking water, 1/4 cup of the remaining pesto, or to taste, and salt and pepper to taste. To make the pesto: In a food processor combine the arugula, the walnuts, the Parmesan or Sardo, the salt, and the garlic and pulse the motor until the walnuts are chopped fine. With the motor running add the oil in a stream and blend the pesto with hot cooked pasta, potatoes, or vegetables. The pesto keeps, chilled, its surface covered with plastic wrap, for 2 weeks. Makes about 2 cups. Serves 4 to 6. Gourmet July 1993