Bring a large pot of well-salted water to a boil. Cook the spaghetti or linguine until al dente according to the package directions, reserve at least 1 cup of pasta water, and then drain.

Cook the chopped bacon in a large skillet over medium heat until crisp. Transfer it to a paper-towel-lined plate and leave about 1 tablespoon of bacon fat in the skillet.

Season the chicken with the salt, black pepper, and optional Italian seasoning. Add it to the skillet and cook for approximately 5–7 minutes, turning occasionally, until browned and cooked through; transfer the chicken to a plate.

Add the minced garlic to the skillet and cook for about 30 seconds. Stir constantly and do not let the garlic brown.

Reduce the heat to low and pour in the heavy cream and whole milk. Stir to loosen the browned bits from the bottom of the pan.

Add the Parmesan, mozzarella, and sharp cheddar a little at a time, stirring until the cheeses melt into a smooth sauce. Do not boil the sauce, because high heat can cause the cream and cheese to separate.

Return the chicken and bacon to the skillet, add the drained pasta, and toss until coated. Add reserved pasta water a few tablespoons at a time until the sauce is smooth and glossy.

Garnish with chopped fresh parsley and serve immediately while the sauce is creamy and the pasta is hot.