1. In a large pot of heavily salted boiling water, cook the spaghetti to al dente. Reserve 1 cup of the pasta water, then drain the pasta.
  2. In a large skillet, cook the pancetta over medium-high heat until crisp, 7-9 minutes.
  3. Add the olive oil, then add the garlic, red pepper flakes, and black pepper and cook until fragrant, about 1 minute.
  4. Add the lemon juice to the skillet, then toss in the drained pasta and toss to coat.
  5. Add the Parmigiano and toss, adding pasta water, a little at a time, to help the cheese coat the pasta.
  6. Add the arugula and toss until it wilts, about 1 minute. Season to taste with additional salt, lots of black pepper, and red pepper flakes.
  7. Sprinkle additional Parmigiano on top and serve.