Cook the spaghetti in a large pot of boiling salted water until al dente. Drain well, reserving 1/2 cup of pasta water.
In a large skillet, cook the diced pancetta or bacon over medium heat until crispy.
Add minced garlic to the skillet and cook for about 1 minute until fragrant.
In a bowl, whisk together the eggs, grated Pecorino Romano cheese, and a generous amount of freshly ground black pepper.
Quickly add the hot drained spaghetti to the skillet with the pancetta and garlic, tossing well to coat the pasta.
Remove the skillet from the heat and immediately pour the egg and cheese mixture over the pasta, tossing quickly and continuously to create a creamy sauce. If needed, add reserved pasta water a little at a time to thin out the sauce.
Serve immediately, garnished with chopped parsley and additional grated Pecorino Romano cheese if desired.