Bring a large pot of salted water to a boil. Add the spaghetti and cook according to package instructions until al dente. Reserve 1 cup of pasta water, then drain the spaghetti.

In a large skillet over medium heat, add the diced pancetta or guanciale. Cook until crispy, about 5-7 minutes. Add the minced garlic and sauté for an additional minute, being careful not to burn it.

In a bowl, whisk together the eggs and grated Pecorino Romano cheese until well combined.

Add the drained spaghetti to the skillet with the pancetta and garlic. Toss to combine, then remove the skillet from heat.

Quickly pour the egg and cheese mixture over the hot pasta, tossing continuously to create a creamy sauce. If the sauce is too thick, add reserved pasta water a little at a time until desired consistency is reached.

Season with salt and freshly ground black pepper to taste. Serve immediately, garnished with additional cheese and chopped parsley if desired.