In a small saucepan over medium heat cook garlic in oil 5 minutes, then remove pan from stove top. Fill the food processor with arugula, loosely packed. Add walnuts and 1/2 of the warm oil and both cloves of garlic to the arugula in the processor. Add nutmeg, salt and pepper to the processor bowl, set lid in place and pulse grind the mixture into a thick paste. Add any remaining arugula to the processor and grind into the paste by pulsing the processor again. Transfer arugula paste to a large pasta bowl. Stir in the remaining oil and the cheese. Taste pesto sauce and adjust seasonings.
While cooking penne to package directions for al dente, watch the time. After about 6 minutes, add green beans to the pasta pot. The beans will cook along with pasta the last 2 or 3 minutes. Drain pasta and beans together in a colander then transfer hot pasta and beans to the pasta bowl with the pesto in the bottom. Toss pasta and beans with pesto for 2 to 3 minutes to combine and evenly coat the pasta and beans with the pesto sauce. Serve immediately.