Make penne and set aside

Trim any roots and thick stems from the arugula

Wash the arugula in several changes of cold water and drain well

There should be about 3 cups packed

Heat the olive oil in a large skillet

Add the red onion and cook, stirring, until tender, about 5 minutes; do not brown

Add the garlic and cook 1 minute

Stir in the arugula leaves and cook, stirring, until wilted and coated with the oil, about 2 minutes

Meanwhile, cook penne until al dente and drain reserving about 1/2 cup of the pasta cooking liquid in a large serving bowl

Add drained pasta to the serving bowl

Add the arugula mixture and toss

Sprinkle with the Parmesan cheese and serve.