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

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

In a large skillet over medium heat, add the diced pancetta or guanciale. Cook until crispy, about 4-5 minutes. Add the minced garlic and sauté for an additional minute until fragrant.

Remove the skillet from heat and add the drained pasta to the skillet, tossing to combine with the pancetta and garlic.

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 black pepper to taste. Serve immediately, garnished with chopped parsley if desired.