Bring a large pot of salted water to a boil. Add spaghetti and cook according to package instructions until al dente. Reserve about 1 cup of pasta water before draining.

In a large skillet over medium heat, add chopped bacon. Cook until crispy and browned, about 5–7 minutes.

Whisk together eggs and grated Parmesan cheese in a bowl. If using garlic, add it here. Season with black pepper and set aside.

Add the drained spaghetti to the skillet with crispy bacon while still hot. Remove from heat and pour in the egg and cheese mixture. Toss everything together vigorously.

If the sauce is too thick, gradually add reserved pasta water until desired creaminess is reached. Adjust seasoning with salt if necessary.