Wash the arugula by placing in a large bowl of cold water; swish gently to remove any dirt. Pull the arugula out of the water, shake gently, and dry the leaves in a colander or salad spinner. Trim any thick stems. Set aside.

Cook pasta in a large pot of boiling water, until it is cooked but still a bit firm to the bite (al dente). Drain pasta, reserving 1 cup of the water from the pot.

Heat the oil in a large skillet over medium-high heat. Add the onions and cook, stirring often, until softened, about 4-5 minutes. Add the garlic and cook, stirring, for 30 seconds. Stir in the lemon zest and juice, salt, black pepper and red pepper flakes.

Add the pasta and ½ cup pasta cooking water to the skillet and cook, tossing gently, for 2-3 minutes until the sauce coats the pasta. Add more pasta cooking water if needed.

Add the arugula and the Parmesan cheese, toss to wilt the arugula, and serve.