Cook your pasta according to the instructions. Right before you drain it, reserve a cup of that starchy pasta water.
While your pasta sits in the colander, put the pot it boiled in back on the stove over medium heat. Add some olive oil, then add the red onions and a couple spoonfuls of capers, if you're using them. When the onion is golden, add in the lemon zest, lemon juice, and 1/2 cup of chicken stock, if you have it. Add as much arugula (or spinach) as you like. I always add more than I think because it wilts down to practically nothing!
Let that wilt down a bit, then add your cooked pasta back to the pot along with a splash more of that pasta water to loosen it back up.
Now, listen to your heart and follow your instincts to finish: add in a big handful of that cheese and grab your tongs and toss that pasta. Add in some of that pasta water too. You want to do this until your pasta looks silky. You will add in splashes of the pasta water as you go along. You can't really add too much cheese or water so don't worry! You can also add in butter at this point, too. Your wrists may hurt but it is worth it.