1. Cook the Pasta:
Bring a large pot of salted water to a boil. Cook the pasta according to package instructions until al dente. Drain and set aside, reserving 1/2 cup of pasta water.
2. Cook the Bacon:
While the pasta is cooking, heat a large skillet over medium heat. Add the chopped bacon and cook until crispy, about 5-7 minutes. Use a slotted spoon to remove the bacon and set it aside on a paper towel-lined plate to drain.
3. Sauté the Onion and Garlic:
In the same skillet, remove some of the bacon grease, leaving about 1 tablespoon. Add the chopped onion and sauté over medium heat until softened, about 3-4 minutes.
Add the minced garlic and cook for an additional 30 seconds until fragrant.
4. Make the Creamy Sauce:
Lower the heat to medium-low. Pour in the heavy cream and bring to a gentle simmer. Stir occasionally, allowing the cream to thicken slightly (about 2-3 minutes).
Stir in the grated Parmesan and mozzarella cheeses (if using), and continue to cook until the cheese has melted and the sauce becomes creamy. Season with salt and pepper to taste.
If the sauce gets too thick, add a little of the reserved pasta water, a tablespoon at a time, to reach your desired consistency.
5. Combine the Pasta and Bacon:
Add the cooked pasta to the skillet with the creamy sauce. Toss to coat the pasta evenly in the sauce. Add the crispy bacon and mix again.
6. Serve:
Plate the pasta and garnish with freshly chopped parsley. If desired, sprinkle with extra Parmesan cheese or red pepper flakes for a little heat.