Bring a large pot of salted water to a boil, add the pasta, and cook for 8 to 10 minutes or until tender yet firm (as they say in Italian, al dente). Drain the pasta well, reserving ½ cup of the starchy cooking water.

Meanwhile, heat the olive oil in a deep skillet over medium heat. Add the pancetta and sauté for about 3 minutes, until it is crisp and the fat is rendered. Toss the garlic and onion into the fat and sauté over medium-low heat for 3 to 4 minutes to soften. Add the red pepper flakes.

Beat the eggs in a mixing bowl and whisk the reserved pasta water into the beaten eggs. Add the Pecorino Romano, stirring well to prevent lumps.

Add the hot, drained spaghetti to the skillet and toss for 2 minutes to coat the strands in the garlic-and-onion mixture. Pour the egg-and-cheese mixture into the pasta, whisking quickly until the eggs thicken—but do not scramble them. Season the carbonara with several turns of freshly ground black pepper, and taste for salt. Mound the spaghetti carbonara in warm serving bowls, and garnish as desired with the additional grated Pecorino Romano.