Dice the guanciale into ¼-inch pieces and cook in a cold skillet over medium-low heat for 10–15 minutes until crispy and golden brown.

Transfer the guanciale to a paper towel-lined plate to drain excess fat, keeping the rendered fat in the skillet.

If using, add minced garlic to the skillet with remaining guanciale fat, sauté for a minute, then remove from heat.

Whisk together eggs, grated Pecorino Romano, grated Parmesan, and black pepper in a large bowl until smooth and creamy.

Cook the spaghetti in salted boiling water per package instructions until al dente, about 8–10 minutes. Reserve 1 cup of pasta water before draining.

Add the hot spaghetti to the skillet with guanciale fat and toss to coat.

Remove from heat, pour the egg mixture over the spaghetti, and toss quickly to combine, allowing the eggs to cook gently with residual heat.

Adjust the sauce thickness by adding reserved pasta water gradually until desired consistency is reached.

Mix in the crispy guanciale, reserving some for garnish. Toss to combine.

Taste and adjust seasoning with salt and black pepper before serving, garnished with cheese and cracked pepper.