Cut the stems off the parley and chop them finely. Chop the leaves separately. Set aside. (This step is optional. If you don’t have parley stems, just skip them).
Bring a large pot of water to a boil. Add a generous amount of salt and the spaghetti. Stir, and start making the garlic olive oil sauce.
To a large stainless steel skillet, add the olive oil, finely chopped garlic, parsley stems, and red pepper flakes. Cook over medium-low heat for about 3-4 minutes, until the garlic just starts to change color.
At this point, add a ladle of pasta water to the skillet. Cook for about 3-4 minutes, and add the spinach. Cook, until spinach has wilted.
While the spinach is simmering, check on the pasta. You should cook the pasta about 1 minute less than you normally would.
Reserve about 1 cup of pasta water and drain the pasta. Add the drained pasta and the reserved parsley leaves to the olive oil sauce. Stir, and keep cooking for about 1 more minute, adding splashes of pasta water, if necessary.
Turn off the heat and plate the pasta. Garnish with freshly ground black pepper.